Health tourism is a complex tourism product that involves a great number of specialized facilities and services for travel motivated by the need to improve health and increase the quality of life. Wellness tourism as one of its components, and especially medical tourism, is an organized implementation of preventive healthcare and curative programs with the purpose to prevent disease, preserve and improve health, the service quality of which affect the diversity of the overall tourism product, and thus the better recognizability of the destination in an increasingly demanding market. The aim of the empirical part of the paper is to analyze the relevant quality concepts of wellness services, and the models and methods of measuring the quality of wellness services. In the empirical part, expectations and perception of service customers were researched by the use of the modified SERVQUAL model, while the applicative contribution of the paper is reflected in the development of guidelines for improving the quality of service provision. The study was conducted at the Thalasso Wellness Centre in Opatija on a random sample of 87 respondents. The results were processed using the standard SPSS 12.00 software package, and descriptive and bivariate statistical analysis were applied as well. The study results confirmed the empirical assumption of the adequacy of using the SERVQUAL model for measuring the quality of wellness services since a clear picture of the overall quality of wellness services was obtained. The results unambiguously point to the existence of a gap between the perception and expectations of customers. Given that the highest average perception ratings went to tangible and reliable dimensions, and the lowest average ratings to compassion or empathy dimensions, the largest gap was also established in these dimensions. The results of the empirical research can serve the management of the institution as an incentive for justifying the continuous research of wellness service quality, the satisfaction of wellness service users and job satisfaction of the wellness centre staff, by using other methods and models as well.