It is shown that the Shuttleworth equation g = gamma + partial derivative gamma/partial derivative epsilon is valid for the interface between a solid isotropic electrode and a liquid electrolyte in the case of an infinitesimal surface deformation only. In the equation above, g is the surface stress, gamma is the interfacial tension, and epsilon is the scalar surface strain variable (elastic surface deformation). In the general case of a finite surface strain, the interfacial tension is a function of two scalar strain variables rather than one. Under conditions of an elastic finite surface strain, the modified Shuttleworth equation is deduced for the trace of the tensor of the surface stress. The Shuttleworth equation above takes into account the existence of both scalar elastic surface strain variables.