The sessile droplet method is used to study the wettability of aluminum oxide (single-crystal Al2O3, i.e. sapphire) by melts of Au-Si, Cu-Si, Ni-Si, Pd-Si, and Ge-Si in relation to silicon concentration, temperature and exposure time. Addition of silicon to melt leads to a fall in the wetting angle from 120–140° for Au, Cu, Ni, Pd and Ge to 70–90° for an alloy with 30–70 at.% Si. The adhesion activity of silicon (a nontransition element) is lower than for transition metals (for example, Ti, Zr). In the systems Au-Si and Pd-Si a phenomenon of dewetting (the wetting angle increases after the initial spreading) is observed. Interphase chemical processes responsible for the wetting angle are studies by mass spectrometry, microscope and profilographic analyses for metal alloys and the surface of solid sapphire.