GaN/AlGaN quantum wells (QWs) grown by molecular beam epitaxy are studied by time-resolved photoluminescence spectroscopy. We compare samples grown on sapphire and on GaN substrates. In the latter case, we observe long-lived, bi-exponential decays, whereas faster, mono-exponential decays are obtained on comparable hetero-epitaxial QWs. Totally different behaviors are measured when the temperature is changed. We interpret our results in terms of secondary feeding processes involving localized states in the barriers. We also discuss the role of dislocation densities. (C) 2001 Elsevier Science B.V. All rights reserved.