Using luminescence upconversion with 100 fs resolution, we have investigated the intersubband scattering of electrons in GaAs quantum wells (QWs). The energy separations between the first and second confined electron subband (E(12)) were more or less than the LO phonon energy (E(LO)) In narrow QWs (E(12) > E(LO)) the intrinsic LO phonon scattering rate is found to be 1.6 x 10(12) s(-1). A Monte Carlo simulation allows us to reproduce the experimental data with high accuracy. In wide QWs (E(12) < E(LO)) We find a very fast thermalization of the carrier population in the QW within times shorter than 300 fs. The intersubband relaxation rate for electrons in the n = 2 subband can be estimated to be shorter than 1.5 ps. (C) 1997 Academic Press Limited