Charge decomposition analysis (CDA) has been applied to study the interaction of CO and NH3 on several metal surfaces. For NH3 chemisorption, CDA predicts a large donation from the NH3 lone pair to the surface and a vanishing contribution for the backdonation. However, for CO chemisorption, CDA predicts the a-donation to be larger than the pi-backdonation, contrary to other methods. CDA overestimates the extent of the sigma-donation because of its large overlap with the metal orbitals compared with the overlap of the pi orbitals. The same situation is found in Ni(CO)(4). Therefore, the CDA method should only be applied to the investigation trends.