Hydrogen overpotentials at nickel sulphide cathodes have been measured in 1 N NaOH. The nickel sulphide catalysts have been examined for their structure and composition using scanning electron microscopy (SEM) and microprobe analyses. Nickel sulphide electrodes exhibit a transient behaviour during cathodic polarization. A significant increase in performance could be noticed accompanied with a clear change in the mechanism of the evolution reaction. Voltammetric measurements show a first anodic peak around -0.95 C and a second broad peak around -0.65 V. On the basis of galvanostatic potential curves, both anodic peaks have been attributed to oxidation reactions involving hydrogen; a strongly bonded hydrogen and a dissolved or weakly absorbed hydrogen. It is the formation of the strongly absorbed hydrogen structure which is believed to be the main cause of the observed transient behaviour of the nickel sulphide electrodes.