The partitioning of alloying elements between a and b phases was measured at different steps of the isothermal transformation at 710 and 610°C in Ti 17 alloy using EDX analyzer in the TEM. In addition, the transformation crystallography was determined. No differences in substitution elements were observed for a same transformation temperature in the a phase, however the composition varied with the transformation temperature. For the partial transformed specimens, gradient in composition were obtained. Results are compared to calculated compositions using ThermoCalc software and Saunders database.