A new adsorption model is developed for small molecules in zeolites whose form is based on features revealed by molecular simulation. Adsorption is assumed to occur onto a 3-D polyhedral lattice, and both the energy and entropy of the lattice sites are accounted for using a statistical mechanics approach. Energetic interactions are described by an Ising model with both 2- and multibody nearest-neighbor interactions. Entropic interactions are included by an adsorption site volume term which accounts for the loss of translational freedom associated with lattice crowding. The model is applied to a system of small molecules (xenon, methane) adsorbed in idealized zeolite NaA, where adsorption has been shown by computer simulation to occur on finite, cuboctahedral lattices (Van Tassel et al., 1992). The model quantitatively predicts the simulated isotherm over the entire pressure range. Comparison is made with a Langmuir model and a van der Waals gas model which, although valid at low pressures, fail at high pressures due to overestimation of translational entropy and inaccurate portrayal of sorbate-sorbate interaction energy.