Systemic intermediaries are organizations that support socio-technical transition processes by bridging structural deficiencies in a transitioning domain. In doing so, intermediaries perform a number of transition-supportive roles. Previous research has identified what these roles are and how intermediaries perform them. However, while transitioning domains are by definition in a state of change, little is known about the dynamics in intermediation activities. That is, how intermediaries develop and adjust their transition-supportive roles, and what kind of organizational conditions enable an intermediary to be dynamic in its support to a transitioning domain. We explore these questions in an exploratory longitudinal case study (2011-2017) of the evolution of the transition service portfolio of a major systemic intermediary from the domain of sustainable energy. Four contributions to the literature on systemic intermediaries are made. First, we operationalize the concept of transition-supportive roles. Second, we explore on the micro-level how intermediaries design support services. Third, we articulate the mechanism by which systemic intermediaries can develop the transition-supportive roles they perform toward transition stakeholders. Fourth, we identify an organizational setup that enables an intermediary to be dynamic in its portfolio of transition services