Hydrogen production from propionate at various concentrations by Rhodopseudomonas capsulata, a purple nonsulfur bacterium, was studied at a temperature of 31 degreesC, a pH of 7.0, and an illumination intensity of 3000 Lux. Among the six levels of propionate, 3.84 g/L was found to be the optimum propionate concentration for H-2 production in terms of substrate utilization efficiency, H-2 percentage, cumulative H-2 production, and H-2 yield. A modified Gompertz equation was able to describe properly the production of H-2 from propionate. A comparative study of H-2 production with acetate, propionate, and butyrate at 40 mM showed that, as a substrate for H-2 production by R. capsulata, propionate was better than butyrate, but less favorable than acetate.