We report on continuous-wave and time-resolved measurements of the photoluminescence of good-quality single quantum wells at low temperatures (4-30 K). Luminescence arising from both free and localized excitons is observed and the influence of excitation localization on the photoluminescence decay time is investigated. Resonant pumping at the light-hole exciton transition is found to greatly increase the generation of free heavy-hole excitons. In this case, the free-exciton lifetime increases linearly with temperature (10-30 K) and the dependence of the slope on the well thickness is found to be in good quantitative agreement with the theoretical model of Andreani et al.