High pressure may strongly influence the energy of intercrystalline or solid/liquid interfaces, their structure and diffusion along them. As a consequence, solid state chemical reactions may depend on pressure. This concerns their kinetics, nucleation, as well as wetting of the surfaces of the crystals by the new growing phase. Some examples are given on the pressure effect on the energy and solid state reactions as well as grain boundary diffusion. It is shown that the pressure effect on the interface energy and kinetic processes strongly depends on the interface structure.