Background: The International Classification of Functioning Disability and Health (ICF) differentiates between functions, activities/capacities, contextual factors and participation. Dysfunctions can result in impaired capacities, which in turn can lead to problems with participation depending on the context. Motivational and volitional deficits are intervening factors. The question is to what degree work performance (i.e. participation), motivational factors, and the inability to perform activities (i.e. dysfunctions) interact. Method: Incapacities were measured in 213 patients (70% women) admitted to the Department of Behavioral Medicine using the Mini-ICF-Rating for Mental Disorders (Mini-ICF-APP), work performance was measured with the Endicott Work Productivity Scale (EWPS), and volitional and motivational problems in regard to work were assessed with the Arbeitsbezogenes Verhaltens- und Erlebensmuster (AVEM). Sick leave prior to admission and work-related problems were assessed in a special clinical interview. Results: The mean global score of the Mini-ICF-APP across all patients was 0.84 ± 0.56 (SD), corresponding to ‘mild disability’. The highest disabilities in this patient population were found for ‘flexibility’ (item 3, 1.64 ± 0.94); the lowest disabilities were found for ‘self maintenance’ (item 11, 0.19 ± 0.44) and ‘mobility’ (item 12, 0.43 ± 0.85). Partial correlations between the Mini-ICF-APP, AVEM and EWPS showed highly significant correlations between the Mini-ICF-APP and EWPS and no or weak correlations between the AVEM and the Mini-ICF-APP or EWPS. Conclusion: Work performance is primarily related to the inability to perform activities and incapacities, and only due to attitudes or volitional/motivational factors to a much lesser degree. Therefore, capacity and motivation can and must be separated.