A new method is described to determine changes DELTAgamma of the specific surface energy of solids from the elastic deformation of a circular plate using Kosters laser interferometry. It was confirmed experimentally that the deformation is the same for normal pressure and for lateral stress. A support of the plate was used which exerts almost no forces on the plate thus ensuring maximum sensitivity of the method. The error of the light path of two beams reflected at a distance of about 10 mm was +/-0.2 nm corresponding to a sensitivity of DELTAgamma = +/- 3 mN m-1. Simultaneous measurements of the changes of specific surface energy, mass and charge for the adsorption of KF and the underpotential deposition of Pb, Tl and Zn on polycrystalline gold are compared to theoretical predictions. In addition to the changes of superficial energy predicted by the Lippmann equation contributions from surface stress are observed which probably originate from structural changes in the surface.