The solvation free energies of MenNH4-n+ (n = 0, 1, 2, and 3) have been computed by means of a combined discrete-continuum model, in which the ionic solute and the solvent molecules in the first layers around the hydrogen atoms or protons bonded to the nitrogen atom are treated quantum mechanically and the remaining solvent is simulated by a continuum model. The results show that the discrete-continuum model can predict the solvation free energies of charged species within the chemical accuracy. On the basis of the computed solvation free energies, we further explore the nature of the irregular order in basicity of a series of methylamines in aqueous solution. Analyses reveal that the short-range solute-solvent interactions play a very important role in the inversion of basicity.