The thermodynamics of solid surfaces is the least developed part of the surface thermodynamics. The Gibbs adsorption equation referred only to fluid systems, and an analogous equation for deformable solids was derived considerably later. Its general interpretation is complemented now by the concept of the tensorial anisotropic chemical potential corresponding to an anisotropic deformation state in a solid. Surface stresses produce the mechanical surface tension of which the direct measurement was a great problem up to now. The first method of measuring surface tension of solids has been proposed very recently and is based on the effect of deformation on solubility.