transition temperature about 91 K that is above the boiling point of liquid nitrogen (77 K). Practical application of YBCO bulk superconductors strongly depends on the presence of effective pinning centres in YBCO crystal structure, which interact with magnetic flux lines. Chemical substitution of the atoms in the YBCO crystal lattice by others chemical elements was used for creating of non-superconducting regions, which are effective pinning centres, [1] and aluminium belongs to the most successfully used elements for achievement of this purpose [2]. It has been found that contamination of YBCO by Al took place, when different oxides such as Y2O3, BaO, BaCO3 or CuO were used for YBCO growing in the Al2O3 crucibles [3]. However, direct reaction of Al2O3 with YBCO has not been comprehensively investigated yet. The main task of our work was concentrated on investigation of Al2O3 powder reaction with commercially synthesised YBCO powder. Different powder mixtures of YBCO and Al2O3 (0wt.% to 12.5wt.%) were investigated. Differential thermal analysis and differential scanning calorimetry simultaneously with thermogravimetric measurements were used for investigation of thermal reactivity of YBCO and Al2O3 powder mixtures. X-ray diffraction and high-temperature X-ray diffraction analyses as well as scanning electron microscopy and energy dispersive X-ray spectroscopy were used for deeper investigation. The peritectic melting temperature of YBCO decreased to about 940°C from 1010°C (pure YBCO) after increasing of Al2O3 concentration. Investigation of the powder mixtures, after additional annealing or fully melting, showed that the first reaction of Al2O3 with YBCO took place below 940°C. Different Al rich secondary phases with Ba, Cu and O were formed and partial substitution of the Cu atoms by the Al atoms was observed. The thermal properties of YBCO were substantially changed with increasing of Al2O3 and after full peritectic melting of the powder mixtures the number of secondary phases decreased and concentration of Al in the YBCO crystal structure increased. From the obtained results it was evident that after full melting of the powder mixtures Al rich secondary phases dissolved and the Al atoms substituted for the Cu atoms in the form of YBa2(AlxCu1-x)3O7-δ composition.