Strained In0.35Ga0.65As/GaAs quantum wells of various thicknesses (6-16 monolayers) obtained by molecular beam epitaxy at 400 degrees C were studied by optical pumping techniques at 4 K. Improved quantum well optical properties, due to significant increase of the critical thickness for the two dimensional-three dimensional growth mode transition, were demonstrated. The luminescence polarization, when observed, decreases very quickly with increasing excitation energy. This implies that very efficient spin relaxation processes between the optically- and non optically-active levels of the photogenerated excitons are involved in such heterostructures. (C) 1995 Academic Press Limited