Electroless Ni-P deposits with phosphorus content ranging from 4.8 to 12.8 weight percent (w/o) were examined using potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) techniques to characterize the effect of phosphorus on the corrosion behavior of electroless nickel-plated mild steel in deaerated 40 w/o NaOH solution. Anodic polarization of the electroless Ni-P alloys in caustic NaOH solution shows that the passive current density decreases with increasing phosphorus content in the deposits. At an applied potential of -1.2 V vs. saturated calomel electrode (V-SCE) (close to their E(corr)), EIS data indicate that the R(ct) for Ni-P alloys in NaOH solution increases with increasing phosphorus content. X-ray photoelectron spectroscopy (XPS) results suggest that the primary constituent formed on the Ni-P surface after EIS measurement in 40 w/o NaOH solution at an applied potential of -0.4 V-SCE (in the passive region) is Ni(OH)(2), which is responsible for the passivity of the Ni-P alloys. The polarization resistance of Ni-P alloys in NaOH solution at -0.4 V-SCE also increases with increasing phosphorus content.