Isothermal torsion tests were performed on a Ti-6Al-4V alloy in the two-phase region. The results show that straining leads to an increase in the beta phase fraction, which increases slightly with strain rate. Transformation took place at 880, 940, 960, 980 and 1000 A degrees C. The extent of this type of dynamic transformation (alpha to beta) was increased when the temperature approached the transus temperature. The reverse transformation (beta to alpha) occurred during isothermal holding after torsion and the volume fraction retransformed increased with time. The driving forces promoting dynamic and reverse transformation together with the energy barriers opposing these transformations were derived and compared. The critical stresses required to initiate dynamic transformation are calculated from the flow curves. This analysis confirms that the peak stresses are always higher than the critical stresses at the temperatures employed in the present tests, which makes it possible for the transformation to occur.