We investigate the spin relaxation of optically excited charge carriers in a variety of GaAs-AlxGa1-xAs quantum wells and superlattices at low temperatures by ultrafast time-resolved photoluminescence spectroscopy. Structures with uniform static optical properties show a markedly varied spin scattering in the time domain, thereby requiring a series of studies with systematically modified epitaxial growth conditions. A multiexponential spin relaxation is seen in quantum wells with a fast initial component of less than 1 ps and a subsequent relaxation within 150 ps. For the same growth conditions, the polarization decay time in superlattices approaches bulk values, indicating a dimensionality dependence of the spin scattering. © 1991 The American Physical Society.