The effect of the iron concentration in the external environment on hydrogen production was studied using sucrose solution and the mixed microorganisms from a soybean-meal silo. The iron concentration ranged from 0 to 4000 mg FeCl2 l(-1). The temperature was maintained at 37 degreesC. The maximum specific hydrogen production rate was found to be 24.0 ml g(-1) VSS h(-1) at 4000 mg FeCl2 l(-1). The specific production rate of butyrate increased with increasing iron concentration from 0 to 20 mg FeCl2 l(-1), and decreased with increasing iron concentration from 20 to 4000 mg FeCl2 l(-1). The maximum specific production rates of ethanol (682 mg g(-1) VSS h(-1)) and butanol (47.0 mg g(-1) VSS h(-1)) were obtained at iron concentrations of 5 and 3 mg FeCl2 l(-1), respectively. The maximum hydrogen production yield of 131.9 ml g(-1) sucrose was obtained at the iron concentration of 800 mg FeCl2 l(-1). The maximum yields of acetate (389.3 mg g(-1) sucrose), propionate (37.8 mg g(-1) sucrose), and butyrate (196.5 mg g(-1) sucros) were obtained at iron concentrations of 3, 200 and 200 mg FeCl2 l(-1), respectively. The sucrose degradation efficiencies were close to 1.0 when iron concentrations were between 200 and 800 mg FeCl2 l(-1). The maximum biomass production yield was 0.283 g VSS g(-1) sucrose at an iron concentration of 3000 mg FeCl2 l(-1).