Zeolites containing gallium in their crystalline framework structure are prepared by treating a zeolite material with a reagent capable of replacing a part of the aluminum of the framework structure of the zeolite material with the gallium. The method is especially applicable for the preparation of faujasitic materials of the formulaM.sub.(x+y/n) [ALO.sub.2 ].sub.x [GaO.sub.2 ].sub.y [SiO.sub.2 ].sub.zwherein:M is a charge balancing ion and n is the oxidation state thereof,x, y and z are the respective numbers of tetrahedra represented respectively by AlO.sub.2, GaO.sub.2 and SiO.sub.2,x+y+z=192, for a said faujasitic structure with no missing tetrahedra,x+y is from 0.1 to 71 inclusive, andy is from 0.01 to 60 inclusive.