A method is described by which the surface tension of surfaces occurring in the systems copper-hydrogen or argon and copper-lead-hydrogen or argon is determined. At temperatures round 850° C the surface tension between copper grains is 640 dyne/cm. and between copper and the equilibrium lead-copper liquid phase 340 dyne/cm. The surface tension of a copper-gas surface is the same whether the gas is hydrogen or argon, within the limits of precision of the experiment, and has the value 1,800 dyne/cm. When lead vapour is present in the gas atmosphere at a partial pressure of the order of 0.1 mm. Hg the surface tension of the copper-gas surface has the much lower value of 780 dyne/cm It is suggested that the reduction is due to the adsorption of lead at the copper-gas surface The failure of the liquid phase to exhibit a contact angle of zero against solid copper surfaces is thereby understood