A technique was developed to study in situ the early stages of the reaction between iron and air. Using a high-temperature microscope, we observed at temperatures between 1000°C and 1050°C and within the first 30 s of reaction, the formation of iron-oxide layers on the surface of low-carbon steel. We observed the nucleation and growth of a first layer of iron oxide and the consecutive formation in sequence, of higher iron oxides sweeping over the surface of the former oxide. The grain boundaries of the steel substrate remain visible for quite some time following exposure to an oxidizing atmosphere indicating that diffusion through steel grain boundaries may have a determining influence on the formation of oxides. These findings emphasize the importance of conducting further studies to better understand the kinetics and mechanisms by which iron-oxide layers form in the early stages of oxidation.