This work presents a complete thermodynamic analysis of the equilibrium and stability of solid-fluid phase boundaries under stress when capillary effects are present. The approach is based on the standard thermodynamic extremum principle according to which the free energy assumes a minimum in stable thermodynamic equilibrium. The results obtained allow for anisotropic surface strain and extend in this respect earlier work by Gibbs. An equation quantifying the combined action of capillarity and nonhydrostatic stress on mineral solubility is derived and applied to determine the relative significance of these effects for pressure solution. It is demonstrated that the phase equilibrium at curved and stressed boundaries is metastable, strict thermostatic stability being attained only when the solid-fluid phase boundary is plane and the stress state in the solid is hydrostatic at the phase boundary. ¿American Geophysical Union 1991