The observed IR spectra of the CH3NO2-.(H2O) and CH3CO2-.(H2O) complexes display sequences of up to seven transitions in the region of the OH stretch fundamentals. This is indicative of strong anharmonic coupling between the OH stretch modes and one or more low-frequency modes. Cubic force fields have been calculated for these two complexes using the MP2 method, and these have been used to calculate the vibrational spectra and to identify the key couplings responsible for the "extra" lines in the observed spectra. In addition, a simple adiabatic model involving the OH stretch and intermolecular rock vibrations is introduced and shown to account in an near quantitative manner for the structure in the OH stretch region of the spectra. (C) 2003 American Institute of Physics.