Hydrogen overvoltage of Ni-S films containing from 12 to 41wt% sulfur was measured in 9N NaOH at a current density of 40A/dm2 over a period of 30 days. Values for films containing 30wt% sulfur or less increased gradually over time to about 220-360mV, but those for films containing 32wt% sulfur or more (amorphous alloy) decreased from 230 to 110-150mV in 3-10 days, remaining constant there after. Such values show an excellent catalytic activity of Ni-S alloy compared with platinum and steel. The sulfur content of films decreased during electrolysis to about 1/3 of the initial value. The lower hydrogen overvoltages obtained for the films with higher sulfur content were due to the formation of a high density and a uniform distributed fine Ni3S2 particles from the Ni-S amorphous alloy during electrolysis.