Classical sessile drop experiment was performed for intensive microstructure and phase composition studies of the reaction product region (RPR) formed because of high temperature interaction between the aluminum-copper alloy (Al-16.7 at.% Cu) and SiO2 (amorphous) substrate. The experiment was performed in vacuum at 1173 K for 2 hour contact time. Scanning and transmission electron microscopy techniques were applied to reveal the details of the complex microstructure of reactively formed product zone at the drop/substrate interface. Three regions of different structure and phase composition were well distinguished in the RPR: the first layer was composed of large-faced Al2O3 crystals of alpha type surrounded by the Al2Cu metallic channels, the second one had the same phase composition but different morphology of the alpha alumina and with silicon as an extra component (dissolved within these phases and as precipitates). The third area revealed very fine-grained (100-200 nm) microstructure, in which Al2Cu and Si grains were embedded in orthorhombic delta-Al2O3 matrix. Moreover, the presence of the deformation twins in silicon, twinned on (1-11) plane was related to large strains present in the area close to the SiO2 substrate and coming from the volumetric mismatch of SiO2 and the freshly formed Al2O3.