In situ X-ray diffraction was used to study the oxide formation on AISI 316L stainless steel (SS) specimens during isothermal oxidation at 900 degrees C in air. Results were compared with those obtained on AISI 304 SS to determine the role of molybdenum on the oxidation process for the AISI 316L SS specimens. Our results show that molybdenum plays a major protective role during steel oxidation. This element is found in a NiMoO4 phase at the internal oxide-metal interface. The high molybdenum content of the alloy hinders the outward diffusion of iron and leads to a lower growth rate and better scale adherence. The oxide scale is then composed of Cr2O3 with a small amount of Mn1.5Cr1.5O4 at the external interface. The improved scale adherence appears to be due to a keying effect at the scale/alloy interface promoted by molybdenum.