Monte Carlo (MC) computer simulations are reported for a realistic model of carbon dioxide adsorbed on the exposed graphite basal plane. Isotherms were calculated using the grand canonical MC algorithm, and energies and densities were evaluated using the canonical MC algorithm. New experimental data for this system are also presented and, together with previous data, compared with the simulations. Excellent agreement is obtained. Auxiliary properties such as the coverage dependence of the average tilt angle (relative to the surface plane) for the molecules in the monolayer were evaluated. In addition, the temperature dependence of the monolayer capacity for this system is obtained directly from the simulations. The molecular areas in the completed monolayer that were obtained from these monolayer capacities are compared with other estimates including those obtained from a BET analysis of the isotherms. It is noted that the observed change in average tilt angle with increasing coverage produces a coverage-dependent molecular area that is not taken into account in the BET or any other known theory of multilayer adsorption.