Phase transitions in Ti-6Al-4V were studied by means of high temperature X-ray diffraction using a resistance heated furnace in high vacuum and Cu K _α radiation. The α and β phases could be observed conveniently using the (10·0), (00·2) and (10·1) peaks of the α structure and the (110) peak of the β. The phase composition of the specimen at selected times and temperatures was determined from spectra recorded in the range 32-45° (20), the scan time being ∼8 min. The technique was successfully used to monitor the kinetics of the α-β transition during heating and the β-α transition during cooling, including the transformation kinetics during isothermal holds. Measurements of the interplanar distance change with temperature for the crystallographic planes of α and β phases enabled derivation of the overall thermal expansion for the alloy using a rule of mixtures.