We report on measurements of the ultrafast changes in the absorption and refractive index of pure GaAs in the near-infrared and midinfrared ranges of the optical spectrum. The multicolor experiments have been performed by using an ultrafast optical parametric oscillator that allowed photoexcitation of the electron-hole plasma, and probing the associated changes in both parts of the dielectric function in a wide spectral range (∼1–4 μm) with femtosecond time resolution. We found that while the change in absorption is primarily due to resonant inter-valence-band optical transitions and, therefore, provides information on the dynamics of nonequilibrium holes, the corresponding refractive index change is dominated by the nonresonant Drude contribution of free carriers. Unlike the dynamics of the absorption coefficient, the time evolution of the refractive index change is found to be strongly affected by the processes of diffusion of free carriers into the bulk of the material and surface recombination. The latter effect may proceed on a picosecond time scale depending on the surface quality of the samples. We deduced from our measurements that the characteristic surface recombination velocity constant may be as high as (Formula presented) © 1999 The American Physical Society.