This review compares the three-dimensional structures of the solublec-type cytochromes that functionally link membrane-bound energy transducingcomplexes in algal, cyanobacterial and eubacterial photosynthesis. Inanoxygenic photosynthesis, cytochrome c2transfers electronsbetween the cytochrome bc1complex and the eubacterialreaction center. Cytochrome c6performs the analogous functionin oxygenic photosynthesis, carrying electrons between the cytochromeb6f complex and Photosystem I. One of the most strikingobservations from recent progress in elucidating the structures of themembrane-bound electron transfer partners of cytochrome c2andcytochrome c6is the structural similarity between PhotosystemI and the bacterial reaction center. Are there similarities in thestructures of cytochrome c2and cytochrome c6that are functionally relevant in the context of their interaction withtheir membrane-bound redox partners? In addition to providing a generaldescription of the structures of cytochrome c2and cytochromec6, the following comparison addresses this question.