A general model of electrical conductance, which takes into account the contribution of various phases encountered during a transformation, is described. The case when more than the initial and the end phases are observed during the resistance measurement is discussed. In particular, the general case reduced to the presence of three phases during the sequence of some phase formation, is considered in the light of available experimental data. The conductance model provides a relationship between the measured resistance and the fraction of the new phase formed. The shapes of the curves derived on the basis of the model are discussed in terms of the relative resistivities of the phases that might be involved in some reaction. Transformations in the Co–Si, Ni–Si, Pt–Si and the (Ti,Ta)Si2 systems are tested and related to the model. The fractions derived by the model on the basis of the suggested method can then be used to evaluate some kinetic parameters such as the activation energy for the formation of a phase of interest. Effective activation energy can be obtained by defining the time needed at each temperature to obtain a constant resistance at a certain fraction of the new phase formed. The activation energies obtained by using the model are in good agreement with the experimental values.