The rate of thiophene decomposition was shown to be independent of the type of chalcogens used in catalysts MoX2/Al2O3, where X = S, Se. On the contrary, the rate of selenophene decomposition was shown to be higher on catalysts MoSe2 than that on MoS2. This observation suggests that the decomposition proceeds on anion vacancies. The decomposition of either thiophene over MoSe2 or selenophene over MoS2 results in the formation of partially substituted chalcogenides. At that, the molar ratios of the substituted chalcogen to Mo were shown to coincide in both cases. The fact that the rate of the thiophene decomposition does not depend on the degree of anion exchange indicates that the decomposition is not associated with hydrogenolysis.