The electrochemical reactions which occur on a rotating disk nickel electrode in a non-buffered aqueous solution of 0.5 M Na2SO4 (5 < pH < 13) are : water oxidation; formation of Ni CII) and low life time Ni (III or IV) soluble species; formation of a black electrochrome NiOOH layer from acid base reaction of Ni (III) or redox reaction of Ni (IV) with water. The oxidation reaction of Ni occurs probably on reactivated parts of the passive electrode as a consequence of a significant local decrease of pH due to high O-2 evolution rate localized on some surface defects. Difficulties of reactivation of nickel explain why NiOOH layer grows lower and lower as the solutions pH are higher. In 1 M KOH solutions, the voltammetric characteristics of the compound synthetized are similar to those of the passive layer of nickel. The oxidized form (NiOOH) of the passive layer of Ni in 1 M KOH forbids further growth of the layer. This behavior can be explained by the layer compacity and by fixed positions of water molecules in the NiOOH lattice. The continuous growth observed for the synthetized layers is due to their porosity, which permits permanent solution-Ni contact.