Type 17-4 PH martensitic precipitation-hardenable stainless steel, having a combination of high mechanical properties and good corrosion resistance is widely used in aerospace, chemical, and petrochemical and food industries This alloy has a high resistance to stress corrosion cracking but age hardening treatment, increases its sensitivity to stress corrosion cracking. There are several works investigating the influence of different aging treatments on the microstructure, mechanical properties and corrosion resistance of 17-4 PH steels, however there are little works studying the simultaneous effects of aging treatments and molybdenum content on corrosion properties of these steels. In this research, the effect of molybdenum on stress corrosion cracking resistance of 17-4 PH alloy using U-bend samples in chloride solutions, as well as its effect on passivity, has been investigated. Quantometer, Scanning Electron Microscope(SEM) and potentiostat were used to determine the chemical composition, microstructure and anodic polarization behavior of the alloys. It is found that molybdenum has a useful effect on stress corrosion cracking resistance under the peak aged conditions, and this is because of development of delta-ferrite phase by increasing the molybdenum content and subsequently decreasing the strength of the alloy. (c) 2006 Springer Science + Business Media, Inc.