The methods of thermal analysis – thermogravimetry, differential thermal analysis – were used to study the catalytic system phosphor-molybdenum heteropoly acid–aluminum oxide. It is shown that the modification of aluminum oxide has a direct effect on the state of the phosphor-molybdenum heteropoly acid. On the surface of ƞ-Al2O3, phosphomolybdic acid is destroyed by interaction with strong Lewis’s acid sites of the carrier, while on γ-Al2O3 it retains the individuality, probably due to multipoint adsorption on weak Bronsted and Lewis acid sites of the oxide