We investigate the exciton formation process from free carriers in a single GaAs quantum well. We focus on the formation processes assisted by emission and absorption of acoustic and optical phonons. The formation rates are evaluated and compared to the experimental results. The exciton dissociation rates are also calculated as a function of the lattice temperature. Finally, these rates are used in a rate equation model in order to reproduce the dynamics of excitons and free carriers in a photoluminescence experiment. We show that for lattice temperature lower than 30K, the thermalization between free carriers and excitons is very slow, and deviations from the action mass law persist for several nanoseconds.