The purpose of this paper is to explore the characteristics and activities of intermediaries that are involved in innovation diffusion and, thus, contribute to the acceleration of sustainability transitions. The paper develops a taxonomy of four types of diffusion intermediaries based on official firm registry data on intermediaries involved in renewable electricity technologies in Sweden, which differ from each other in terms of business activity width and sectoral coverage. An analysis of the intermediaries business descriptions shows that: (1) most intermediaries are not exclusively involved in intermediation and many are active in several sectors, (2) the relative importance of different intermediation activities varies between different types of intermediaries, and (3) diffusion intermediation has a technology-specific component. These findings imply that researchers and policy makers should not treat diffusion intermediaries as a homogeneous collective. Moreover, there might be an (unexploited) potential for intermediaries and policy makers to exploit cross-sectoral learning.