This study examines the interaction of sulfur-based oil additives on steel. Sulfurized isobutylene, dialkyldithiocarbamate and sulfurized esters were the additives investigated in this report. For the first time, X-ray absorption near edge structure (XANES) spectroscopy has provided detailed insight into the chemistry of both the thermo-oxidative and tribochemical films generated from these additives. It was found that the chemical nature of these films was strongly dependent on the operating environment for the additives. The XANES revealed that thermally, all three S additives reacted very similarly with steel to form a film mainly comprised of iron sulfate at temperatures as low as 100 °C. The ample supply of diffused oxygen from the base oil along with oxide naturally present on the substrate allowed for the complete oxidation of the S from the decomposed additive to iron sulfate. Tribochemical films were comprised of different forms of sulfur than observed for the thermo-oxidative films. The moderate AW conditions yielded a mixture of both oxidized and reduced forms of sulfur, with pyrite, FeS2, being the major constituent. Rubbing between the steel pin and the substrate partially depleted the oxide layer present, allowing the additive to interact intimately with the fresh substrate, yielding FeS2. Under extreme-pressure conditions, complete removal of the oxide layer occurred with a drastic increase in the interfacial temperatures between the pin and v-block, allowing for the complete thermal decomposition of FeS2 to FeS to occur. AFM imaging of the AW films revealed the presence of tiny smooth domains randomly oriented, which were completely different from the pad-like structures observed for AW films generated from a typical ashless thiophosphate additive. The inability of the sulfur-based additives to form large pad-like structures, which can ultimately support the load, resulted in poor AW protection to the metal.