Purpose: As the effect evaluation of our randomized controlled trial the “Comprehensive Approach of Reintegration for clients with Multiple problems” (CARm) showed no superior effect on re-integration into paid employment of the clients when compared with clients of the care as usual, we conducted this process evaluation to gain insight into whether the intervention was conducted according to protocol. Methods: Using questionnaires on recruitment, reach, dose delivered, dose received, fidelity, context, and satisfaction we collected data from 40 labour experts of the Public Employment Service of the Dutch Social Security Institute, and from 166 disability benefit recipients dealing with multiple problems. Results: Only few of the labour experts provided the key elements of the intervention to their clients. Between the clients of both groups were no significant differences in the dose received. More than half of the labour experts of the intervention group reported organizational changes. Conclusion: The lack of effect of the CARm intervention was almost certainly caused by implementation failure. Once again this study showed the importance of involving all stakeholders in developing and the conduct of the intervention, and of clarifying the consequences for the organization, to ensure that it can be conducted according to protocol.