Results are presented which indicate that the measured peak position in inelastic electron tunnelling spectroscopy (IETS) is dependent, for certain vibrational modes, on the polarity of the bias voltage. Reproducible peak shifts on bias reversal of 2.2+or-0.5 mV and 0.30+or-0.05 mV occur for the O-H stretching mode in undoped junctions and the 144 mV mode in benzoate-doped junctions, respectively. A simple model, involving perturbation of mode energy by the large electric field present in the tunnel junction, is suggested. The results agree satisfactorily with the predictions of this model in the case of the O-H mode, but it has not proved possible to make a similar comparison for the benzoate modes due to a lack of anharmonicity data. The extent to which such models rely upon a detailed knowledge of the molecular environment, and the possible consequences of the 'built-in' barrier electric field on IET spectra are briefly discussed.