A comparative study of the basic character and of the chemical reactivity of the oxide site of the MgO and CaO (100) surfaces has been performed on the basis of ab initio cluster model calculations. Two molecules, CO2 and SO2, have been chosen as probes of the basic character of the O2- surface site. The latter was modeled by a OM(5) cluster, with M = Mg or Ca, embedded in the proper Madelung field. We found that CO2 and SO2 exhibit completely different reactivities with the two surfaces. On MgO, the two probe molecules form a weakly bound surface complex, while on CaO, we observe the formation of strongly bound sulfite and carbonate species. The reason for the different reactivities has been analyzed in detail. A decomposition of the interaction energy into electrostatic, polarization, and charge transfer contributions shows that CaO is a better base than MgO. This is also the reason why CO2 and SO2 are more strongly bound on the heavier oxide. The ultimate reason for the different surface reactivities of MgO and CaO, however, is not of chemical nature and can be simply explained in terms of electrostatic stabilization of the surface anion. The O2- ion at the surface is stabilized by the Madelung potential of the ionic crystal. This is smaller in CaO than in MgO, thus leading to a higher basicity and reactivity of CaO. In this respect, a regular surface site of CaO behaves similarly to a low-coordinated defect site of MgO, We also show that there is a direct relationship between the lattice constant of the ionic crystal and the surface basicity. This may be relevant in the context of the acid-base properties of strained oxide thin films grown on metal substrates.