Vibrational force fields for model compounds of unprotonated and protonated Schiff bases are obtained by nb initio molecular orbital (MO) calculations. The C=N stretching frequency shifts downward upon protonation in the calculations at the Hartree-Fock level, contrary to the experimental results. The effect of electron correlation on the C=N stretching force constant and its basis-set dependence are investigated. Calculation at the second-order Moller-Plesset perturbation (MP2) level with the 6-31G*(*) basis set (the 6-31G* basis set augmented by one set of polarization functions on the Schiff base proton) is necessary for correctly calculating the change in the C=N stretching force constant upon protonation. The effect of hydrogen bonding between the Schiff base proton and its counterion is also studied. Hydrogen bonding increases the frequency shift occurring upon HD exchange of the Schiff base proton (Delta(HD)), in agreement with previous studies. This increase in the value of Delta(HD) originates not only from strong mixing between the C=N stretch and the N-H in-plane bend in the protonated species but also from mixing between the C=N stretch and the N-D stretch in the deuterated species.