A thermodynamic approach based on mass, energy, and entropy balances is developed for capillary condensation hysteresis in porous media, Processes of fluid condensation and evaporation are considered as quasi-isothermal irreversible transitions between equilibrium states of the capillary system. The corresponding balances express the equilibrium interfacial properties of the system through the work and heat measured in capillary process and, thus, may be used for evaluating the vapor-condensed phase interface area as a function of pressure, surface area of the porous solid, its pore size distribution, and surface fractal properties.