The kinetics of martensitic transformation induced by a tensile stress pulse (generated by the reflection of a compressive shock wave at a free surface) in time durations in the microsecond range, were studied in an Fe-32wt%Ni-0.035wt%C alloy. The tensile hydrostatic component of stress interacts with the dilatational strain (~0.04) of the martensitic transformation and increases the M s temperature. Shock waves were produced by normal impact of a projectile on a target in a one-stage gas gun. Impact experiments were conducted by varying either the temperature (−10 to −50°C), or pulse duration (0.22−1.76 μs) at a constant pressure. The martensitic transformation, normally considered to be athermal in Fe-Ni-C alloys, exhibits an isothermal nature in the microsecond regime. The fraction transformed increases with decrease in temperature at a constant pulse duration, and increase in pulse duration at a constant temperature. The mean volume of the lenticular martensite was found to be constant throughout the progress of the transformation, consistent with the autocatalytic spreading of clusters. The activation energies for γ→α' transformation in the Fe-32wt%Ni-0.035wt%C alloy, calculated with a modified version of Pati and Cohen's kinetic equation [ Acta metall. 17, 189 (1969)], range from 38,000 J/mol at −10°C to 25,000 J/mol at −60°C. The activation energies are linearly related to the total driving force (chemical free energy change + mechanical work due to the transformation). The activation volume for the transformation was calculated and found to be equal to approximately 60 atoms (0.7nm 3). This indicates that the martensitic nucleation in this alloy, and under the imposed stress conditions, is interface-mobility controlled.