Crystalline transition metal aluminates conforming generally to the formulaMA.sub.a.sup.v Z.sub.b.sup.v.nAl(OH).sub.3.mH.sub.2 Owhere M is a transition metal having a valence charge of +2 selected from the group consisting of Cu, Zn, Mn, Fe, Co, and Ni, where AZ represents negative valence ions or radicals, v is a negative valence of 1, 2, or 3, n is a value to provide a mol ratio of Al/M of at least 1/1, preferably at least 1.5/1, and m is an integer of from zero to the maximum for waters of hydration, with (va)+(vb) equal to 2, are prepared in alkaline aqueous medium wherein transition metal compounds are caused to form adducts with amorphous hydrous alumina, Al(OH).sub.3, said adducts forming crystals when heated, said aluminates being useful, e.g., as ion exchangers and as spinel precursors among other things.