The classical problem of acid strength of the hydrogen halides in aqueous solution is revisited by means of the RISM-SCF/MCSCF theory, an ab initio electronic structure theory combined with the statistical mechanics of molecular liquids. Foe energy changes associated with the chemical equilibrium HX+ H2O reversible arrow X-+ H3O+ are studied for a series of hydrogen halides (X: F, Cl, Br, I). The free energy differences between hydrogen halides and the dissociated anions are mainly governed by the subtle balance of the two energetic components-formation energies of hydrogen halides and solvation energies of the anions. It is shown that hydration structure around hydrogen fluoride is qualitatively different from the other three hydrogen halides. The well-known specificity of the hydrogen fluoride with respect to the acidity in aqueous solution is explained in terms of the characteristic hydration structure, Molecular geometries and electronic structures of the solute molecule as well as the solvation structure and free energy components are also discussed in detail. The old concept of electronegativity proposed by Pauling is reexamined on the basis of the modern theoretical approach.