The influence of surfactant on the solvent vapour pressure and equilibrium states in open capillary systems has been considered both in the presence and absence of gravity. The relationships have been derived describing a change in vapour pressure in the course of surfactant adsorption in closed one-phase and two-phase capillary systems. The stability conditions for such systems are discussed as related to surface tension, surface elasticity, surfactant adsorption and concentration, and the interfacial curvature.