The cooperativity between coadsorbed molecules on the MgO surface was investigated with the ab initio molecular orbital (MO) method using (MgO)(4) and (MgO)(6) clusters as models of the catalytic surface. On the pair of three-coordinate sites, the H-2, NH3, or H2O molecule is chemisorbed heterolytically, like H--Mg-O-H+, H2N--Mg-O-H+, or HO--Mg-O-H+. When the two adsorbate molecules are cochemisorbed on the adjacent set of four three-coordinate sites, the total chemisorption energy is much larger than the sum of chemisorption energies of each molecule. The extra stability, i.e., the energy cooperativity, is almost the same, around 20 kcal/mol, regardless of the adsorbed species, as far as both of them are heterolytically chemisorbed. The H-2-CO coadsorbed system, in which CO is adsorbed nondissociatively, shows no cooperativity. The cooperativity above is partly due to the electrostatic attraction between the coadsorbed ionic species and partly due to the enhanced charge transfer (CT) interaction. Based on these results, it is suggested that higher coordination sites, inactive for chemisorption by themselves, may be activated by preadsorption at the neighboring pair of three-coordinate sites and may actually be "the active catalytic site" on the MgO surface.