A significant increase in the C-O stretching force constant (F-CO) and a decrease in C-O bond length (r(CO)) result upon coordination of carbon monoxide to various cationic species. We report a study designed to elucidate the factors responsible for this effect. In particular, we distinguish between an explanation based on electrostatic effects and one based on withdrawal of electron density from the 5 sigma orbital of CO, an orbital generally considered to have some antibonding character. Ab initio electronic structure calculations on CO in the presence of a positive point charge (located on the carbon side of the bond axis) reveal that a simple Coulombic field increases the C-O stretching force constant and decreases the bond length. Coordination of CO to a simple cationic Lewis acid such as H+ or CH3+ is calculated to increase F-CO (and decrease r(CO)) to extents slightly less than those engendered by a point charge at the same distance from the carbonyl carbon. These results indicate that electron donation from the 5 sigma orbital has no intrinsic positive effect on the magnitude of F-CO.