A model has been developed which provides a fairly simple rationale for interphase precipitation in the stoichiometrically balanced Fe-V-C and Fe-V-N-C steels. The model assumes that interphase precipitation takes place by the steady-state migration of a stepped interface. Quantitative relationships between ledge velocity, ledge height, transformation temperature and carbide volume fraction, are obtained. The influence of growth kinetics upon the formation of interphase precipitation is also briefly discussed.