Polarization curves, electrochemical impedance measurements and SEM analysis were used to study the dissolution and the inhibition of nickel–phosphorous alloys in 1 M H2SO4. The alloy specimen was obtained by electroless plating on copper substrates. The inhibiting effect of triphenyl-alkyl (C6)-phosphonium (P3AP) was examined. The shape of the polarization curves remained essentially unchanged by addition of the inhibitor, but the current density values decreased significantly in both the anodic and the cathodic domains. The impedance spectra at open circuit potential obtained for various immersion times showed that the dissolution of this alloy was characterized by three capacitive loops. The loops were barely distinguishable at short immersion times, but, in contrast, they appeared clearly when the specimens were left in aggressive medium for one day. Tafel extrapolation in the cathodic region gave a corrosion inhibition efficiency of 90%. In the anodic domain, the current plateau decreased approximately 100 fold as a result of inhibitor addition. The phenomenon leading to the formation of this plateau is more likely related to a film growth process than to a diffusion limitation.