Measurements of electrical resistance provide the easiest way to assess the evolution of reactions leading to the formation (by reaction or phase transition) of a new solid phase. Measurements carried out in situ during heating allow one to see at once that evolution is a function of temperature. The relationship between the measured resistance and the amount of the new phase formed is not direct so the derivation of kinetic data from resistance measurements is not obvious. This paper is an attempt to correctly relate sheet resistance measurements to the amounts of phase formed, and furthermore obtain the activation energy for phase growth. Exact relationships are complex and rather difficult to use, so simplifications have to be made which are justified either through mathematical derivation or actual numerical calculations. Two examples are considered : (1) the diffusion limited planar growth of a phase, and (2) the transformation of a film from one phase to another, assuming that the nuclei are pre-existent and that the film is sufficiently thin for the transformation to occur entirely via the lateral growth of the nuclei. For this latter case, it is shown that there is no linear relation between the sheet resistance and the evolution of the transformation. Using the results obtained from in situ measurements at different heating rates m and taking equivalent values of the sheet resistance, i.e. for equal amounts of the newly grown phase let us say 50% or 80% transformation, the activation energy can be obtained as the slope of lnm/T-2 versus 1/T.