The effects of hydrogen in type 310 stainless steel on the disorder of passive films and pitting susceptibility have been investigated. II is found that hydrogen decreases the breakdown potential, facilitates the pitting initiation and increases the pit density and growth rate. The passive films are found to exhibit n-type semiconductivity. Observed hysteresis in the capacitance-potential curve and Urbach rail in the photocurrent spectra indicate that the passive films are highly defective. Hydrogen increases the disorder of the passive films, which is one of the reasons that hydrogen increases the pitting susceptibility. (C) 2000 The Electrochemical Society. S0013-4651(99)12-048-2. All rights reserved.