Electrodeposited NiSx alloys have been studied using electrochemical techniques, SEM with EDX, XRD and TEM with electron diffraction. The results show that maximum catalytic activity towards the hydrogen reaction is obtained with alloys containing between 25 and 30 at% S. The catalytic activity decreases extensively on either side of the catalytic maximum. SEM pictures show cracked surfaces. Density measurements of the NiSx, films show that the density of the catalytic active films varies between 3 and 8 g cm(-3), depending on the S content. The alloys are amorphous towards XRD. However, when using TEM and electron diffraction, nano crystals of pure Ni are found. The nano crystals are "floating" in an amorphous phase containing Ni and S. The catalytic activity is tested in a 1 M KOH solution. It is found to bo time-dependent. A maximum in catalytic activity is reached after approximately 3 h. Thereafter, the rate of the hydrogen evolution decreases, and after 13 h, approximately half the maximum rate is obtained. SEM pictures after prolonged hydrogen production show a widening in the cracks and a decrease in the S content, indicating that S is leaching out of the alloy, resulting in a decreased activity.