Powders produced by abrasion of steels in a wear-testing machine were collected and subjected to X-ray examination. The abrasion products consisted of mixtures of the parent metal (ferrite) and iron oxide. The oxides observed were FeO and Fe3O4, but never Fe2O3. Small amounts of cementite Fe3C were also present in certain cases. The relative amounts of α-iron, FeO and Fe3O4 in the wear powder depended characteristically on the composition of the steel, its phase constitution, and certain experimental factors. The method is illustrated by its application to a series of carbon steels. The amount of α-iron and the types and amounts of oxides present are indicative of the temperature attained during wear when the known phase relations in the iron-oxygen system are utilized. One aspect of the wear process is considered as being in effect a localized short time heat treatment and `quench' of both the original and abraded test material.