Cobalt complexes are well-known catalysts for photocatalytic proton reduction in water. Macrocyclic tetrapyridyl ligands (pyrphyrins),and their Co-II complexes emerged in this context as a highly efficient class of H-2 evolution catalysts. On the basis of this framework, a new tnacroc-yclit Co-II complex consisting of two keto,bridged bipyridyl units (co diketo-pyrphyrin) is presented. The complex is synthesized along a convenient route, is well soluble in water, and shows high activity as a water reduction catalyst (WRC) In an aqueous system containing [Ru(bpy)(3)]-Cl-2, as a photosensitizer and NaAscO as a sacrificial electron donor, turnover numbers (TONs) of 2500 H-2/Co were achieved. Catalysis is terminated by a limited electron supply and decomposition of the photosensitizer but not of the WRC, highlighting the distinct stability of Co diketo-pyrphyrin.