We performed high-pressure experiments on the crystallization of water ice I and III in the ammonia–water (NH3)x(H2O)(1−x) system, and apply the results to the interiors of icy bodies in the Solar System. Phase equilibrium lines between an entirely liquid solution and a liquid solution in which water ice forms (liquidus lines) were determined for ammonia concentration by mass X equal to 0.034, 0.0472, 0.111, 0.176, and 0.229. Growth–melting of ice I as well as ice III crystals were observed. Application of the results to icy satellites that are potential bearers of ammonia shows that ammonia admixture decreases the depth of the liquidus surface. A shift of the liquidus temperature within a satellite depends on three parameters: the ammonia concentration, X; the temperature gradient, α; and the product of density and gravity, ρg.