Monte Carlo simulations of the hydration of Ca2+ and Mg2+ were performed. The interaction potential used includes polarization and nonadditivity, factors that have been reported to be critical for the proper description of hydration. The effect of cell size, amount of configurations needed to attain equilibrium, and size of the statistical sample were studied. We validated the simulation by obtaining agreement with experiments. A microscopic analysis of the hydration shows that Mg2+ and its first shell can be considered as a supermolecule imbedded in the solvent. For Ca2+ this character is less marked and its first shell responds to the solvent more easily. From this view we explain the coordination properties of both ions. We found that the differences in the coordination properties of both ions are due to a great extent to the differences in the water depletion zone after the first hydration shell. An analysis of the energetic contributions of each hydration shell to the total enthalpy of hydration was made as well as an analysis of local densities to understand electrostriction.