A direct comparison of adsorption of CO and ethylene on AgCl and CuCl is made. An ab initio molecular orbital study using the effective core potential is performed to determine the bond energies and the nature of the bonds between the adsorbates and adsorbents. Experimental results show that both CO and C2H4 adsorb more strongly on CuCl than on AgCl. However, CO adsorbs much more strongly on CuCl than on AgCl, while the difference in heats of adsorption is far less for C2H4 on these two sorbents. Ab initio molecular orbital calculations correctly predict the trends. The natural bond orbital (NBO) theory is used to explain the results. The NBO results show that in the two-way pi-complexation bonding, the d-pi* backdonation plays a major role in determining the bonding for these systems.