In the field of phase transformations in steels, much attention has been paid to the transformation of austenite into diverse product phases but, until recently not much work has been done on the formation of austenite during heating. There are few published models dealing with the transformation of eutectoid or hypoeutectoid steels with a starting microstructure which is a mixture of ferrite and pearlite. The aim of the present work was to use phase transformation theory to develop a model for austenite formation which takes into account the chemical composition and microstructure of the steel studied, and thermal history experienced. Classic nucleation theory and diffusion-controlled growth equations are used to determine the progressive transformation of the different phases into austenite. A phase transformation model with sound physical basis as the one presented in this work can be used to determine the effects of various parameters in the reaction involved, like microstructure ( grain size, pearlite spacing), composition, heating rate and others. Another direct application of this model is the generation of CHT ( continuous heating transformation) diagrams for specific steels, which are a useful reference in research, as well as in many industrial processes. (C) 2003 Kluwer Academic Publishers.