The aim of this work is to evaluate the influence of heating rate and initial microstructure on the anisothermal formation of austenite. In this sense, the start (Ac₁) and finish (Ac₃) temperatures of austenite formation have been determined on dilatometric curves obtained at various heating rates in steels with ferrite and/or pearlite initial microstructures. As it was expected, Ac₁ and Ac₃ temperatures rises linearly with heating rate, except for steels with a pure ferrite initial microstructure where the Ac₁ temperature is almost insensitive to heating rate over the range studied. Experimental results in steels with a pearlite and ferrite-pearlite initial microstructures also show that the elevation of the critical temperatures with heating rate is quite sensitive to the morphology of pearlite. It seems that the higher the heating rate is, the stronger the influence of morphology on the critical temperatures are. This experimental study and the knowledge of the mechanisms that control the austenite formation process have allowed to establish the variables that most directly influence this reaction in steels with pearlite and ferrite-pearlite initial microstructures. Those are the heating rate and the two parameters that characterise the morphology of pearlite, the mean true interlamellar spacing and the edge length of the pearlite colonies interface in pearlitic steels, together with the volume fraction of pearlite and the mean free distance of pearlite in ferrite plus pearlite initial microstructures. Likewise, two equations have been proposed for the determination of the start (Ac₁) and (Ac₃) finish temperatures of austenite formation as a function of those variables.