Kinetic studies of hydrolysis and deuteriolysis of microcrystalline 2‐methyl‐4H‐benzoaxazin‐4‐one 1 into N‐acetylanthranilic acid 2 are presented. Hydrolysis of mixtures of microcyrstalline 1 and 2 are given. The results are interpreted and a mechanism is proposed for the solid gas reaction. A theoretical approach of the crystal packing of 1 permits an evaluation of the mode of diffusion of water molecules in the crystal. Both experimental and theoretical results lead to a proposal of the mechanism of water propagation and the hydrolysis process in the solid phase.