A 1N solution of in O18‐enriched water was used to study the passivity of nickel by means of nuclear microanalysis. The fixation of the first monolayer of oxygen coming from the water was shown to have a decisive effect on the passive nature of the metal; however, further thickening to a maximum of five layers also influences the anodic current through an ion transfer mechanism. The oxygen loss rate of the passive layer, measured in open circuit and under polarization near steady‐state conditions, was found to be equal to ∼1013 atoms/cm2−·sec. The current efficiency for passive film formation was calculated and found to be low. It is suggested that passive layer growth occurs at the metal‐film interface.