The effects of volatile fatty acid (VFA) compositions on the cell growth, H-2 production rate and H-2 yield have been investigated using mathematical models. The results show that a modified logistic model indicates that the maximum cell growth of 0.03 h(-1) was obtained at acetate fraction of 0.571, propionate 0.143 and butyrate 0.286 in the total amount of VFA. The response surface methodology appears to be a useful approach to select appropriate substrate compositions for H-2 production. Under the conditions in this work, the fractions of 0.551-0.566 for acetate, 0.204-0.211 for propionate, and 0.230-0.238 for butyrate were optimal for H-2 production by Rhodopseudomonas capsulata. (c) 2005 Society of Chemical Industry.