Adsorption, the equilibrium between two-dimensional and three-dimensional fluids, is considered in the framework of Ono-Kondo lattice theory. A critical point correction to Ono-Kondo theory is proposed, and new approximations for binodals and spinodals are derived. These approximations describe the phase behavior near the critical point more accurately including two-dimensional and three-dimensional binodals. They also improve the prediction of adsorption behavior.