The biological hydrogenation of organic compounds using sulphate-reducing bacteria was investigated, A high pressure stainless steel bioreactor and an atmospheric pressure reactor were constructed so that identical reactions at different pressures could be compared. The mesophillic sulphate-reducing bacteria Desulfovibrio desulfuricans (ATCC 7757) were used. Qualitative tests showed that Desulfovibrio desulfuricans was able to retain its viability at hydrogen pressures up to 35 atmospheres at a temperature of 37 degrees C when exposed to fumarate. Evidence of fumarate dismutation was observed since half of the fumarate was converted to succinate. The effect of bacterial cell concentration was also studied under hydrogen gas. For a doubling of the bacterial cell (reactivated) concentration, the conversion of fumarate doubled from 40% to 80% in 12 h, A first-order reaction rate model was fit to the fumarate concentration data. The first-order reaction rate constant was found to be a Linear function of the bacterial cell concentration.