The effect of phase transformations induced in the surface layer of alumina ceramics during its direct joining with copper activated with oxygen or titanium on the mechanical strength of the ceramic/copper joints was examined. The materials used in the experiments were an alumina single crystal, alumina ceramics (97.5 wt% Al2O3), the cermet mixtures: Cu-Cu2O with 10-50 wt% of Cu2O, copper with 5 wt% of Ti, and copper with 5 wt% of Ti and 10 wt% of Ag. The microstructure of the transition layer was examined by the X-ray diffraction method (XRD), scanning electron microscopy method (SEM) and energy dispersive x-ray spectroscopy (EDX). The mechanical strength of the joints was measured using the three-point bending method. The amount of oxygen optimal for the joining process was determined. It has been demonstrated that the cohesion of the joints depends not only on the formation of the individual phases but also, or even primarily, on the microstructure of the transition layer formed between them.