The problem of intersubband relaxation in GaAs/GaAs quantum wells, where the energy separation of the two lowest subbands is smaller than the optical phonon energy, is considered. Time resolved pump-and-probe measurements are performed with a far-infrared free-electron laser on two multiquantum well samples with similar thicknesses (˜300 Å), but different doping and mobilities. The measured lifetimes are shorter than could be explained by acoustic phonon emission alone. Monte-Carlo calculations show the importance of electron-electron scattering for thermalization of the hot electron distribution function and subsequent optical phonon emission from the long thermal tail. © 1996 Academic Press Limited.