Thin-foil X-ray energy-dispersive chemical analysis of an (α+β) titanium alloy has been used to establish the compositional profiles which develop between theα andβ phases as a result of heating into theβ field at a rate of 100 K sec−1. The results show thatα phase transforms toβ via a diffusional mechanism as it is progressively enriched inβ-stabilizing solutes. Complete diffusional transformation ofα toβ occurs 160 K above theβ transus but theβ phase remains chemically inhomogeneous. On subsequent quenching at a rate of 400 K sec−1,β phase containing less than approximately 10wt% Mo equivalent of β-stabilizing solutes transforms to martensite. The proposed transformation mechanisms are consistent with the results of resistivity measurements made during the heating and cooling cycles.