Background: In long-term care, persons with dementia are often cared for in specialised facilities, which are rather heterogeneous in regard to their design and care concepts. Little information is available on how these facilities and care concepts bring about changes in the targeted outcomes. Such knowledge is needed to understand the effects of care concepts and to consciously shape further developments. This study aimed to explore the mechanisms of impact of a specific care concept from a dementia special care unit and the contextual aspects that influence its implementation or outcomes.Methods: Using a qualitative approach to process evaluation of complex interventions, we conducted participating observations and focus groups with nurses and single interviews with ward and nursing home managers. Data were collected from two identical dementia special care units to enhance the contrasts in the analysis of two non-specialised nursing homes. We analysed the data thematically. We conducted 16 observations, three group interviews and eleven individual interviews.Results: We identified seven themes in three domains related to mechanisms that lead to outcomes regarding residents’ and nurses’ behaviour and well-being. The development of nurses' skills and knowledge changes team-level competence and leads to an altered understanding of nursing. The promotion of a positive work climate reduces distress and promotes the long-term implementation of the care concept. Adjusted spatial structures and personnel strategies facilitate the implementation of interventions for residents and promote the fulfilment of their needs. Personalised psychosocial interventions promote residents' relaxation, engagement in activities and social interaction and thereby empower them to become part of the social community, to spend their time purposefully, to have positive experiences and to experience belonging and affection. The implementation and outcomes of the care concept are influenced by contextual aspects relating to the (target) population and cultural, organisational and financial features.Conclusions: The care concept of the dementia special care unit results in higher levels of relaxation, activities, and social interaction of residents. Its implementation highly depends on the shared understanding of nursing and the skills of the nursing team. Changes in residents’ characteristics result in altered effects of the concept.Trial registration: DRKS00011513