The standard hydration enthalpy, DeltaHdegrees(hyd), of a monatomic cation was calculated as the sum of (1) the enthalpy due to the long-range interaction between a hydrated ion and bulk water, DeltaHdegrees(LR), (2) the enthalpy due to the short-range interaction between the ion and water molecules in the first hydration shell, DeltaHdegrees(SR), and (3) the enthalpy due to the ligand field stabilization of an ion, DeltaHdegrees(LF), which arises for a transition-metal ion. DeltaHdegrees(LR) was estimated on the basis of the Born theory assuming the radius of the hydrated ion as the interatomic distance between the ion and the oxygen atom of a water molecule in the first hydration shell, r(M-O), determined experimentally. DeltaHdegrees(SR) was evaluated on the basis of the donor-acceptor interaction between an ion and a water molecule coordinating to the ion, which was evaluated by the molecular orbital calculation of a monohydrated cluster of an ion combined with the Mulliken population analysis. DeltaHdegrees(LF) was calculated on the basis of the crystal field theory. Hydration enthalpies of 48 monatomic cations thus calculated agreed well with those observed experimentally.