Using the technique of in situ photoprecipitation, a comparative study of the structurally homologous ionic species hexachloroplatinate ([PtCl6]2-), hexachloroosmiate ([OsCl6]2-), and hexachloroiridates ([IrCl6]2- and [IrCl6]3-) was conducted for photoconversion to metallocatalysts for photosynthetic hydrogen evolution. As with earlier [PtCl6]2- studies, [OsCl6]2- can be photoconverted to a metallocatalyst at photosystem I (PSI), although at a rate about 50% slower than that of [PtCl6]2-. However, once photoprecipitated, its catalytic action for H-2 production was 3 times as high as that of metallic platinum. Simultaneous photoevolution of O2 and H-2 was observed in [OsCl6-2-photoprecipitated thylakoids. Maximum hydrogen evolution rate was 113 nmol-h-1.mg chl-1. Surprisingly, neither [IrCl6]2-- nor [IrCl6]3-treated thylakoids were able to produce H2. Analysis indicated that [IrCl6]2- was able to accept only one electron by transformation to [IrCl6]3- which was completely unable to acquire subsequent electrons from PSI. The inability of [IrCl6]2- to be reduced to metallic iridium is presumably due to a high energy level barrier of [IrCl6]3- reduction.