The aim of this paper is to evaluate the possibility of quantitatively monitoring the microstructural evolution in titanium alloys during isothermal heat treatment using electrical resistivity measurements. Specifically, the model alloys Ti-10Mo and Ti-15Mo are studied to investigate the transformation sequences β → β + α and β + ωath → β + ωiso, respectively. To this end, we have combined quantitative in situ resistivity measurements, in situ high energy X-ray diffraction (HEXRD) analysis and microstructural characterisation using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). During annealing at 650 °C of the Ti-10Mo alloy, the microstructure consists of micron-scale α-phase lamellae in the β-matrix, and we show that the increase in resistivity follows that of the α-phase volume fraction. During annealing at 350 °C of the Ti-15Mo alloy, the microstructure is composed of nanometric precipitates of ω phase and we show that the resistivity evolves in a non-monotonic manner, although the fraction of ω always increases. These results cannot be explained by simple models where the description of the microstructure is limited to the phase fraction and suggest the importance of the interfacial contribution to the resistivity in these alloys.