Abstract Since the discovery of bainite, research over many decades has revealed a substantial amount of information about the mechanism of the bainite transformation in steels. Elements of the theory are now routinely being used in many parts of the world in the design of novel alloys and in the interpretation of a variety of experimental data. However, current experimental and theoretical understanding is limiting technological progress. The purpose of this atom probe tomography study was to track atom distributions during the bainite reaction in a nanocrystalline steel. The results are providing new experimental evidence on subjects critically relevant to the understanding of the atomic mechanisms controlling bainitic ferrite formation, such as the incomplete transformation phenomenon, the carbon supersaturation of ferrite, and the plastic accommodation of the surrounding austenite.