Eight NH4+, Mo5+, and Ca2+-modified Al phosphates were prepd. by the reaction of (NH4)(2)HPO4, H3PO4, CaCO3, (NH4)(6) Mo7O24 center dot 4H(2)O, aq. NH4OH soln. (concn. 25% by mass) with fresh Al(OH)(3) in glass or microwave reactor (pH 4-8, time 25-30 min) and studied for chem. and phase compn. The aq. exts. of the phosphates were used as corrosive medium for studying the C steel corrosion resistance by potentiometty. The AlPO4, CaHPO4 and CaCO3 phases-contg. phosphates showed the highest effectiveness in corrosion protection.