The hydrations of Na+ and K+ were investigated by means of Monte Carlo simulations with refined ab initio based potentials. These interaction potentials include intramolecular relaxation, polarizability and many-body nonadditive effects. Care was taken to ensure proper convergence of the MC runs and that the statistical samples were large enough. As a result, agreement was attained with all experimental data available for the hydration of the ions. The water molecules in the first hydration shell were found to have the same intramolecular geometries and dipole moments as those of the bulk. Furthermore, their dipoles were not aligned to the electric field produced by the ion, but quite tilted. The hydration number for the sodium was found to be 5 or 6 water molecules, whereas the potassium's hydration number had a probability distribution ranging from 5 to 10. From an analysis of the energetic contributions of each hydration shell to the total enthalpy of hydration we propose that the hydrated ions have a distinct behavior. Sodium has a stronger interaction with its first hydration shell than potassium, giving the latter a more flexible structure. (C) 2003 American Institute of Physics.