The effect of thiourea (TU) on H-2 evolution has been studied in 1.0 M Na2SO4 + x mM TU (x = 0, 5, 15, 50) at different pH (from 2-8) on Pt electrode by using electrolysis. The theoretical and experimental discharge potentials of the systems have been determined. The amounts of hydrogen gas produced at different times on the cathode at a constant potential(5 V) were measured and the hydrogen yield was calculated. From the data obtained, it reveals that the highest yield of H-2 is obtained, when 1.0 M Na2SO4 + 50 mM TU solution was used at pH = 8. (C) 1998 International Association for Hydrogen Energy.