For many years, service quality has been a field of high interest in scientific and professional debates due to the significant benefits and competitive advantages that a high quality of service can bring to organisations in various economic sectors. In order to be able to properly analyse, evaluate and improve service quality, it is necessary to measure it appropriately, which includes taking into account the specific characteristics of the economic sector in which the analysis is conducted. Much of the scientific work in the field of service quality is based on the disconfirmation paradigm, suggesting that service quality can be operationalised as the gap between customers' expectations and perceptions. This theoretical framework is used in this thesis as well. An implication of this approach is that the user's perspective represents the core of the service quality assessment. Service quality is often conceptualised as multidimensional, but the dimensions of service quality are not the same in each environment. It is therefore important to determine the specific factors that define service quality in the area of interest. The main goal of this doctoral thesis is to analyse, understand and measure the determinants of service quality in ecotourism, a sector that has grown steadily in recent years at the global level. The richness and preservation of natural resources are a necessary but not sufficient condition for the development of diverse and high-quality ecotourism. This can be achieved by setting up or upgrading a service quality infrastructure that meets the expectations and needs of eco-tourists while at the same time contributing to the economic, ecological and social aspects of ecotourism. To do this, organisations providing ecotourism services need to understand which factors are important for determining service quality in ecotourism in order to be able to better assess the strengths and weaknesses of their product. In addition to the analysis of service quality in ecotourism, this thesis investigates a secondary issue which is the impact of service quality on image destination and future behavioural intentions. Such relationships, although potentially important, represent an under-researched area in the context of ecotourism. Based on the limited existing research, it can be suggested that this analysis can contribute to a better understanding of the potential impacts of service quality in the area of ecotourism. Despite ecotourism being a growing segment of tourism as a whole, the number of papers which comprehensively deal with service quality in ecotourism and the abovementioned related concepts is somewhat limited, especially in Europe. Furthermore, no comprehensive empirical measurements of service quality in all eight national parks of the Republic of Croatia have so far been carried out. Since service quality plays a significant role in further developing ecotourism, this research gap can be considered a problem. National parks represent one of the main touristic attractions in the Republic of Croatia and constitute an important part of domestic ecotourism. They are therefore a valid environment to examine service quality in the Croatian ecotourism sector. In addition to the thematic gap, this thesis also fills a methodological gap: so far there has been no measurement of service quality in all eight Croatian national parks by using the ECOSERV model. Based on the above, the main scientific hypothesis of the doctoral dissertation was formulated: using scientific conceptual and empirical knowledge about service quality in ecotourism, the dimensions which affect the service quality in national parks as part of ecotourism can be determined, and a model that can contribute to the effective monitoring and measurement of service quality can be suggested, with the additional aim of determining the influence on the destination image and future behavioural intentions as well as contributing to a higher level of service quality in Croatian tourism. In addition, 10 auxiliary research hypotheses were developed. The existing literature on quality, service quality, ecotourism, image destination, and future behavioural intentions has been analysed. The role and importance of service quality in general, as well as in ecotourism, is stressed, and in this context, the specific features of ecotourism services with an emphasis on the sustainability element are presented. Since the concept of service quality represents one of the main interests of this thesis, a further review of models that conceptualise service quality is provided, with special focus on SERVQUAL and ECOSERV models. Based on the literature review, the thesis proposes a conceptual model with the aim of better understanding and empirically measuring service quality in ecotourism. Using a modified multidimensional ECOSERV model as an instrument, the thesis examines the expectations and perceptions from the perspective of the users and evaluates the service quality gap. This modified ECOSERV model expands on the work of Khan (2003) by including the measurement of service quality perceptions and additional service quality attributes related to the sustainability features of services. Furthermore, the thesis assesses differences in the evaluation of service quality related to the socio-demographic characteristics of visitors. The impact of service quality dimensions on the overall perceived service quality, image destination and future behavioural intentions is tested, as is the influence of image destination on visitors' future behavioural intentions. The model was tested in the setting of national parks in the Republic of Croatia to examine service quality in domestic ecotourism, first in a pilot phase and then in the main study. Data for the main study were collected in all eight Croatian national parks using a questionnaire consisting of six sections including expected and perceived service quality, overall service quality, the eco destination image, future behavioural intentions and the socio-demographic profile of the respondents. A total of 541 valid questionnaires were collected from national park visitors. Subsequently, an empirical analysis was carried out using univariate, bivariate and multivariate parametric statistical methods. The reliability and content validity of the instrument was tested and confirmed. A descriptive analysis was used to provide an overview of the respondents’ socio-demographic profile. In addition, the expectations and perceptions of respondents were analysed and a descriptive gap analysis was performed. The highest visitor expectations in terms of service quality were related to the use of park facilities, as well as to the staff who were expected to help. For perceptions, visitors rated highest the sense of security in the national park and visually appealing natural attractions. A negative gap was identified for all the measured items, indicating that the visitors had higher expectations than the perceptions of service quality. This is not an uncommon result in service quality literature and it should be pointed out that the overall service quality was assessed as relatively high, i.e. the visitors of the national parks seemed to perceive the overall quality of services quite positively. The dependent t-test confirmed that expectations differed in a statistically significant way from perceptions. To identify the existence of differences between socio-demographic groups in view of their service quality expectations, perceptions, and the corresponding ECOSERV gap, an independent t-test and analysis of variance (ANOVA) were used as part of the bivariate analysis. The results show that for most of the socio-demographic characteristics there is no clear pattern. The main statistically significant differences were found for the perceptions of service quality by different age groups. The exploratory factor analysis based on the service quality gap resulted in a 7-factor solution which supports the multidimensional structure of the service quality concept as suggested by the modified ECOSERV model. However, it does not fully match the initially proposed structure. The identified dimensions of service quality are: “Empathy and responsiveness”, “Local influence and appearance”, “Reliability”, “Learning”, “Embeddedness in nature, and attractions”, “Sustainable practices” and “Competence”. The identified model was further validated with reliability and validity tests. Furthermore, the results obtained from the multivariate regression analysis suggest that the dimensions of service quality have an effect on the overall service quality, image destination and future behavioural intentions, although the statistical significance and size of the effects vary across the dimensions. Finally, the positive impact of image destination on future behavioural intentions is confirmed. After discussing and linking the results to previous research, the thesis concludes with a summary of its conceptual, methodological, empirical and applicative contribution. Finally, a description of the limitations of this work is provided and several potential avenues for future research are suggested.