The effects of hydrogen on the breakdown of passive films formed on Type 304 stainless steel were investigated. Cyclic potentiodynamic polarization measurements revealed that hydrogen decreases the breakdown and protection potentials, indicating an increase in pitting susceptibility. An increase in pitting susceptibility due to hydrogen was confirmed by immersion tests and potentiostatic measurements: hydrogen in passive films increases pit density and decreases the initiation time for the film breakdown. The critical chloride concentration for the film breakdown decreases with the charging current density. Hydrogen also hinders the repassivation process once the film is broken down. Several possible reasons are postulated to explain the effects of hydrogen on the film breakdown.