A Monte Carlo simulation study of the force between two adsorbing walls is described. The adsorbate is in equilibrium with either a bulk gas or a liquid phase. The force is correlated with the adsorption, singlet and radial distribution functions, all of which vary with the distance of separation of the walls, when this is of the order of a few atomic diameters. When the bulk phase is a gas, a new type of surface transition occurs in which the distance of wall separation is one of the determining degrees of freedom.