AbstractThe additivity of the contributions of HOD, H2O and D2O to the contours of the OH and OD stretching bands was studied in a 1:1 H2O‐D2O mixture. The observed non‐additivity of contours was explained in terms of Fermi resonance changes caused by the variations in resonance intermolecular coupling parameters due to isotope dilution. The integral intensities of OH and OD bands in the mixture were addit