Relationship quality is a key concept of relationship marketing that builds long term and economically sustainable relationship. Although numerous studies confirm the importance of this concept, there is still a small number of studies in the professional services sector, despite the continuous growth and importance of this economic sector. Due to dynamic changes in the environment professional services firms often must search for possible collaborations outside their own borders to ensure an effective and customized service which demonstrates the need for shifting the focus in business relationships from a bilateral to a network relationship perspective in value creation. How to design an adaptive and flexible service system and how to improve cooperation for value co-creation in a service network are some of the more recent research priorities in the service sector. Thus, the main aim of this thesis is to, via the social exchange and social interdependence theory and the serivce-dominant logic, investigate the mechanisms through which the interpersonal relationship quality directly, and indirectly through cooperation, leads to the project success of professional services firms. For the purposes of empirical research the exploratory qualitative research through in-depth interviews was performed followed by quantitative research with 209 architects and civil engineers. A positive effect of relationship quality on the project success was confirmed, to which co-providers of professional service contribute at the individual level. The focus was on the success of specific projects in which a service provider cooperated with other co-providers to generate a project solution for the end user. Additionally, the indirect effect of the relationship quality through cooperation on the project success was confirmed. The results of the study also demonstrated the moderating influence of emotional intelligence on the link between cooperation and the project success. Also, the results show that emotional intelligence and relational energy have an exclusionary effect concerning the level of cooperation between co- providers of professional service. The main contribution of this thesis is reflected in the investigation of indirect and moderating effects which interpersonal relationship quality has on the project success. The practical contribution refers to the area of management of professional service firms and project management. Additional theoretical and practical contributions of this doctoral thesis refer to the area of relationship marketing in the construction sector.