Abstract The interactions of a gaseous phase with a liquid metallic surface are analysed in terms of surface tension, adsorption and chemical reactivity. It is shown that thermodynamic equilibrium considerations are not sufficient to interpret surface tension measurements especially at high temperatures, and that diffusion and interactions in the vapour phase must also be considered. Surface tension isotherms are discussed and models allowing calculations of maximum adsorption and surface tension values are presented. These are of particular interest when dealing with systems forming very stable oxides. The influence of surface thermodynamic quantities on chemical reactivity is also shown by means of classical thermodynamic reasoning. One of the main results presented here is that reactions favoured in the bulk phase may not take place at the surface, where the presence of the excess free energy term may favour another reaction, thermodynamically impossible in the bulk phase.