We have investigated the optical‐pumping properties ofn‐type GaAs and the modification of these results produced by compensation of the crystal by diffusion of copper acceptors. The rate of circular polarization and its decrease as a function of a transverse magnetic field have been measured for the various luminescence lines. We obtain the values of lifetime &tgr; and spin‐lattice relaxation timeT1of conduction electrons. WhereasT1does not change upon copper introduction, &tgr; is found to be decreased by two orders of magnitude because copper acts as a killer center. The compensation of the crystal can be characterized in a straightforward manner from the study of the luminescence polarization as a function of energy in the vicinity of the band gap. Finally, the inhomogeneities of the crystal and the local strains can be investigated respectively from the measure of the nonlorentzian tail of the depolarization curve in a transverse magnetic field and of the polarization of acceptor‐related luminescence. All these results show that optical‐pumping techniques can allow simple nondestructive characterization of semiconductors.