The details of the initial relaxation of optically injected hot carriers in polar semiconductors are studied by Raman scattering from nonequilibrium LO phonons and anti-Stokes hot luminescence. Experiments on intrinsic and doped GaAs reveal carrier-phonon and carrier-carrier interaction times, the wavevector dependence of the hot phonon distribution, and the influence of holes on the LO phonon lifetime. Studies in AlxGa1-xAs probe the influence of alloy disorder on the generation of hot phonons. In addition, because there are two optic phonon modes in AlxGa1-xAs, we can experimentally measure the effect of ionicity (the frequency difference between LO and TO phonons) on the generation rate for the hot phonons. © 1988.