The influence of initial pH of the culture medium on hydrogen production was studied using sucrose solution and a mixed microbial flora from a soybean-meal silo. Hydrogen production was not observed at pH values of 3.0, 11.0 and 12.0 but low production was observed at pH values 5.0 and 5.5. The pH of the experimental mixture decreased rapidly and produced hydrogen gas within 30 h. Methane was not detected at initial pH values between 6.0 and 10.0. The sucrose degradation efficiency increased as the initial pH value increased from 3.0 to 9.0. The maximum sucrose degradation efficiency of 95% was observed at pH 9.0. The maximum specific production yields of hydrogen, VFAs and alcohols were 126.9 cm(3) g(-1) sucrose (pH of 9.0), 0.7 gCOD g(-1) sucrose (pH of 8.0) and 128.7 mgCOD g(-1) sucrose (pH of 9.0), respectively. The relationship between the hydrogen ion concentration and the specific hydrogen production rate has been mathematically described. The best kinetic parameters on the specific hydrogen production rate were K-OH = 1.0 X 10(-7) Mol dm(-3) and K-H = 1.1 X 10(-4) mol dm(-3) (r(2) = 0.86). The maximum specific hydrogen production rate was 37.0 cm(3) g(-1) VSS h(-1).