Abstract This paper discusses recent experimental results in the liquid-vapour critical region of mercury. These results show that the existence of the metal-non-metal transition noticeably influences electronic, thermodynamic, structural and interfacial features. Optical reflectivity experiments were conducted for mercury in contact with an optically transparent sapphire substrate. The experiments were carried out with the mercury close to the vapour-liquid coexistence curve. The temperature and density dependence of the reflectivity reveal clearly the existence of a prewetting transition of mercury on the sapphire substrate. At high temperature, the transition line terminates at a surface critical temperature T sc lying below the bulk critical temperature T c . At low temperature, it terminates at the wetting transition temperature T w . Furthermore, the transition lies close to the bulk vapour-liquid coexistence curve and occurs when the bulk vapour pressure is within a few percent of its saturation value.