We have measured the subpicosecond optical response of a solid‐state, semiconductor‐to‐metal phase transition excited by femtosecond laser pulses. We have determined the dynamic response of the complex refractive index of polycrystalline VO2thin‐films by making pump‐probe optical transmission and reflection measurements at 780 nm. The phase transition was found to be largely prompt with the optical properties very close to the high‐temperature metallic state being attained within about 5 ps. The equilibration of the metallic state after femtosecond excitation was modeled by non‐exponentially decaying perturbations in the metallic state electron density and collision frequency. The decay of both these plasma parameters was well fit by a 1/&sqrt;ttime dependence. This indicated that a diffusion process governed the equilibration of the metallic phase of VO2. ©1996 American Institute of Physics.