Mid-infrared spectra of the argon-solvated X − · H 2O, X − · HDO, and X − · D 2O complexes (X=I, F) are reported in both the OH and OD stretching regions. Due to the asymmetric H-bonding arrangement adopted by the monohydrates, two isomers are possible for the mixed isotopomer (X − · DOH and X − · HOD). The spectra establish that the isomeric form with ionic H-bonding to the OD group is preferentially formed for the iodide complex (I − · DOH), while the fluoride ion binds preferentially to OH (F − · HOD). The behavior of the iodide monohydrate is traced to the zero-point contribution from the ion–molecule, out-of-plane bending vibrational mode, while the very strongly red-shifted, anharmonic OH stretches appear to overcome this effect in the F − · HOD complex.