The potential energy surfaces for Ar-CO and He-CO were calculated at the fourth order Moller-Plesset perturbation theory and analyzed using perturbation theory of intermolecular forces. Both the complexes reveal only one minimum related to the approximately T-shaped geometry. For Ar-CO, our best ab initio estimates of R(e) and D-e are 3.70 Angstrom and 496 mu hartrees, respectively, and the optimal angle Rg-com-O is about 80 degrees. For He-CO, our best R(e) and D-e are 3.4 Angstrom and 100 mu hartrees, respectively, at the optimal angle Rg-com-O of 70 degrees. Our geometrical parameters agree very well with the experimental data. Our ab initio well depths are estimated to be within +/-5% in error and are expected to be the most accurate in the literature so far. The D-e values were obtained with extended basis sets which included bond functions. Basis set effects on the dispersion and electrostatic correlation terms that are caused by bond functions were also analyzed. Both complexes are bound by dispersion forces, but the anisotropy of the interaction is determined by the exchange repulsion component. This anisotropy may be interpreted in terms of the relative concentrations and depletions in the diffuse region of the CO charge density. The electrostatic and induction effects proved to be negligible.