Fe-18Cr alloys with additions of 2, 4, 6, and 8% Mo are studied in 1N H2SO4 at 25 C (77 F). The effect of Mo on potential-current density curves is determined potentiostatically. Increasing Mo content causes the critical current density to decrease, and the open circuit potential to become more noble. Alloys with 8% Mo do not show any anodic dissolution peak and passivate spontaneously. Some theoretical considerations of the passivation mechanism are discussed in connection with additional electrochemical measurements.