In today's competitive marketplace, relationship marketing has become an important factor. Companies want to achieve customer loyalty through sustainable long-term relationships with customers, especially in the service industry. Thus, companies implement loyalty programs, and conduct consumer research to evaluate the success of these programs. The customer satisfaction construct, is often used by companies to measure the level of satisfaction of their customers. However, previous literature argues that this construct conceal negative emotions. Therefore, the authors have chosen to investigate one type of negative emotion, namely frustration, within frequent flyer programs. The purpose of this thesis is to explore and identify what incidents cause frustration for members within the SAS EuroBonus program. The authors’ goal is to tap into customer frustration, as well as different types of sensations and the underlying incidents that create these sensations. This study will further investigate what impact customer frustrations can exert on program members’ behaviour towards the SAS EuroBonus program. The research method chosen for this thesis was of qualitative nature; the primary data were collected using a single case study, with semi structured interviews, conducted with members of the SAS EuroBonus program of Silver level or higher. The findings from this research reveal that even though individuals claim to be satisfied, negative emotions and concealed frustration is evident. The incidents found in this case study concern qualification barriers, inaccessibility, no additional value, additional costs and service. These incidents resulted in both strongly and weakly felt frustration sensations, which in turn led to three categories of frustration behaviours, namely avoidance, protest and passive behaviour.