Using a lattice-gas model with pairwise interactions, we study the ordered structures, coverage dependence of the heat of adsorption, and other experimentally observable behavior of adsorbed CO overlayers on Pd(100) single crystal surfaces. Transfer matrix and Monte Carlo methods give accurate information regarding the lattice-gas model that often contradicts simple mean-field-like analysis. We demonstrate the usefulness of the model by reproducing experimental results over a large range of pressures and temperatures. (C) 2004 American Institute of Physics.