A thermodynamics-based model to predict the martensite start temperature of β titanium alloys is presented. The tendency of the β phase to transform into martensite is estimated by applying the Ghosh-Olson approach to martensite nucleation modelling. The energetics and M_s temperature for binary alloys are predicted with good accuracy. A succinct formula predicting M_s temperature in multicomponent β alloys is presented. The new equation is compared to the classical Mo equivalence criterion for assessing β stability.