Mid-infrared spectra of the argon-solvated X- . H2O, X- . HDO, and X- . D2O complexes (X = 1, 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. (C) 2004 Elsevier B.V. All rights reserved.