The formation of gamma-AlON from Al2O3 and AlN was investigated as a function of some of the reaction parameters, that is, temperature and time. The appearance of phases was followed in conjunction with the change in lattice parameter to reveal the reaction sequences of formation. An oxygen-rich gamma-AlON phase formed first, which reacted further with AlN. Nitrogen diffusion through the gamma-AlON lattice seemed to be rate controlling.