Abstract. Background: The efficacy of inpatient psychosomatic rehabilitation in Germany can be considered proven. However, a significant number of patients with mental disorders remain unsatisfied with rehabilitation or being non-responders. Illness beliefs, as the core element of the Common-Sense Model of Self-Regulation (CSM), are widely known as predictors of various health outcomes. Yet, little is known about treatment beliefs as an extension of the CSM and their impact on health outcomes. Aim: As treatment beliefs differ in relation to the treatment, this study aimed to explore rehabilitation-related treatment beliefs in psychosomatic rehabilitation patients with mental disorders before inpatient admission. Thus, knowledge of the concept of rehabilitation-related treatment beliefs will be added. Methods: N = 10 semi-structured telephone interviews were conducted in September/October 2018. A purposive sampling approach was chosen based on the criteria gender, age, and diagnosis. Participants were asked about their rehabilitation expectations starting with an open narrative question. Interviews were analyzed using qualitative content analysis. Results: Participants had a mean age of 48.3 years ( SD = 9.42); five participants were female and five male. 9 main themes with 32 subthemes as components of the rehabilitation-related treatment beliefs were identified: reasons for rehabilitation, conditions within the clinic, rehabilitation planning, organization of the rehabilitation, the content of the rehabilitation, results of the rehabilitation, concerns, expectations toward one’s behavior, and contact to other patients. Limitation: Our sample was too small to analyze the data for different subgroups. Conclusion: Results show that rehabilitation-related treatment beliefs are multidimensional, addressing different aspects of psychosomatic rehabilitation.