Duplex aging is one of the common heat treatments in titanium alloys. The microstructure introduced in the first-step aging has an effect on the growth/dissolution of alpha in the second-step aging. In the present work, a beta + alpha.sub.acicular microstructure is preset in Ti-55531 (Ti-5Al-5Mo-5V-3Cr-1Zr wt%) alloy. The isochronal and isothermal phase transformation kinetics in the second-step aging is studied by combining the dilatometer test with microstructure characterization and local composition mapping. The phase transformations and corresponding temperature ranges are determined as beta right arrow alpha.sub.acicular 643-845 K and alpha.sub.acicular right arrow beta 845-1130 K by isochronal annealing. A TTT diagram for isothermal transformation kinetics is plotted based on the transformed phase fraction and reproduced by Johnson-Mehl-Avrami theory. The calculated kinetic curves are in good agreement with experiment ones. The C-shaped TTT curves verify the classical nucleation and growth of alpha in the second-step aging. In comparison with Ti-55531 alloy with preset beta + alpha.sub.lath microstructure (in authors' previous work), the alpha precipitation exhibits prolonged incubation period and slowed average transformation rate, which is evidenced by a right shift of C-curves for the alpha precipitation portion along the time axis. However, the C-curves of alpha dissolution show a left shift on the TTT diagram. The precipitation kinetics of alpha aciculae from dilatometry is synchronous with that obtained from the diffusion of Al detected in STEM mapping, while the diffusion of slow-diffusion elements lags behind the structural transformation. The TTT diagram and the dataset of microstructure features obtained in the present work can be employed to optimize processing in duplex aging.