Significance The mantles of icy planets comprise large amounts of water, ammonia, and methane ices. To understand their interior structure, it is crucial to study these ices at the extreme pressure conditions they likely experience. Hitherto, such studies have mostly been restricted to individual ices and not considered formation of stable mixtures. We survey here mixtures of water and ammonia and show that high pressures stabilize ammonia hemihydrate, through a transformation from a molecular crystal into a fully ionic solid that involves complete deprotonation of water. We suggest that ammonia-rich hydrates can precipitate out of any ammonia–water mixture at sufficient pressures and are an important component inside icy planets.