We have used photoluminescence measurements to investigate the dynamic coupling between excitons and cavity modes when a quantum well is embedded in a microcavity. The experiments were carried out in quantum wells placed at either the antinodes or the nodes of the optical field in a 3λ/2 cavity, in a broad temperature range (200-6K) and using magnetic fields up to 7T. We find that light is emitted by the exciton-cavity system before it has time to thermalize. The lack of thermalization is most dramatic at low temperature, where the luminescence spectrum shows two peaks of comparable strength separated by 6meV, attributed to emission from the low- and high-energy branches of the coupled system.