The acid-basic properties of cobalt, nickel and magnesium (2%) ions modified aluminum oxide are studied by electronic spectroscopy and diffuse reflection by the benzene and chloroform adsorption. The acid properties of cobalt, nickel and magnesium containing (2%) systems are established to be close. Unlike those, after incorporation of significant magnesium amount (10%) the spinel MgAl₂O₄, in which all cation vacancies are occupied, is formed on the carrier surface so the defectiveness of surface and therefore the samples acidity decrease. This difference can be explained by the specific affect of the transition metals with partially filled d-orbitals on the system acidity