The surface structure of Ga2O3-Al2O3 catalysts was investigated by TR and ESR spectroscopy. As a result of modification by gallia, alumina hydroxyl cover changes significantly. The new band 3680-3688 cm(-1) ascribed to bridging structure Ga-OH-Al has been observed. By means of IR spectroscopy, adsorbed CO and ESR with anthraquinone as a probe molecule, two types of Lewis acid sites and their concentration were defined. These sites are coordinately unsaturated aluminium cation Al-cus(3+) and coordinately unsaturated gallium cation Ga-cus(3+). In the case of Ga2O3-Al2O3 catalysts with a gallia content of 1-10 mol%, surface Lewis acid properties are in accordance with the assumption of solid solution formation. Only under significantly non-equilibrium conditions (large excess of dopant - 20 mol% Ga2O3-Al2O3, relatively low temperature of calcination -720 K, short time of interaction - 4 h) it becomes possible to stabilize highly dispersed state of gallium oxide, for which the characteristic feature is high concentration and local regulation of Ga-cus(3+) on the surface. it was shown that on the alumina surface there an favourable (more than in the case of pure gallia) conditions for the Ga-cus(3+) formation.