Abstract Crystal structures and their implications for function are described for the energy transducing hetero-oligomeric dimeric cytochrome b 6 f complex of oxygenic photosynthesis from the thermophilic cyanobacterium, Mastigocladus laminosus, and the green alga, Chlamydomonas reinhardtii. The complex has a cytochrome b core and a central quinone exchange cavity, defined by the two monomers that are very similar to those in the respiratory cytochrome bc 1 complex. The pathway of quinol/quinone (Q/QH2) transfer emphasizes the labyrinthine internal structure of the complex, including an 11×12 Å portal through which Q/QH2, containing a 45-carbon isoprenoid chain, must pass. Three prosthetic groups are present in the b 6 f complex that are not found in the related bc 1 complex: a chlorophyll (Chl) a, a β-carotene, and a structurally unique covalently bound heme that does not possess amino acid side chains as axial ligands. It is hypothesized that this heme, exposed to the cavity and a neighboring plastoquinone and close to the positive surface potential of the complex, can function in cyclic electron transport via anionic ferredoxin.