0.1 M sodium chloride, proton irradiation resulted in a 220 mV increase in the pitting potential of stainless steel 304 L (0.425 vs 0.644 V saturated calomel electrode). In addition, the passive region of the polarization curve during irradiation was associated with a drop in metastable pitting activity by a factor of 100. Mott-Schottky experiments in pH 1.6 H2SO4 found that irradiation was associated with an increase in oxygen vacancy concentration (V-O = 2.94 x 10(21) vs 3.41 x 10(21) cm(-3)). However, electrochemical impedance spectroscopy experiments found that the Warburg coefficient (sigma) increased during irradiation (47 k Omega cm(2)/s(0.5) vs 118 k Omega cm(2)/s(0.5)). An increase in film impedance was also observed. Given that sigma is inversely proportional to VO, one would expect that an increase in VO would result in a decrease in sigma. This apparent dichotomy, an increase in oxygen vacancies in the space charge region at the film/solution interface and a corresponding increase in sigma, can be explained if the film is composed of inner Cr-rich p-type and outer Fe-rich n-type semiconducting layers. It is proposed that changes in the inner Cr-rich layer of the oxide are responsible for the observed increase in pitting potential during irradiation. (c) 2008 The Electrochemical Society.