Electrical resistance measurements provide a convenient way to study the kinetics of transformations in isothermally annealed thin films. The measured resistance and the amount of a new phase formed during some sort of reaction in a transformation process are often related by a linear function. This simple relationship between the measured resistance and the fraction of the new phase formed is not always valid and the reliability of the results might be questionable. In this communication, a different approach is suggested, which is based on conductance. The validity of the model is tested by experimental data for the Ti-Ta-Si system. The suggested and the linear resistance models are compared, and it seems that it is more appropriate in certain cases to use the conductance approach for kinetic purposes. An effective activation energy can be derived from the results by defining the time needed at each temperature to obtain either a constant resistance, or alternatively, a certain fraction of the new phase. The activation energy derived for the end phase, i.e. (Ti,Ta)Si-2 formation from the experimental data in the example (Ti-Ta-Si) considered is similar to2.6 eV.