Free solution thermodynamics is shown to be inadequate for describing membrane equilibria, The membrane imposes external forces on the permeating solution which can be of potential and/or frictional character. At equilibrium only the potential membrane-solution interaction remains, and it is the cause of pressure, concentration and electrical potential differences on the two sides of the membrane. Such interaction can be allowed for when we formulate a thermostatic description of virtual displacement within the membrane that incorporates the momentum conservation principle in the form of the mechanical equilibrium equation. The thermostatic description that emerges is the same as the one that results from irreversible thermodynamics in the limit of zero dissipation. The description allows formulation of the balance of forces for individual cases of membrane equilibria and transport. It also states that, at equilibrium, the gradient of mechano-chemical potential of each species is equal to zero. The new potential has been given a thermodynamic definition; in the case of electrolytes it becomes the electrochemical potential. The proposed description has been applied to Donnan, osmotic. imbibition and sedimentation equilibria.