A joint spectroscopic-computational study was carried out of molecular librations in ice. The measured Fourier transform infrared spectra included H2O and HDO isolated in D2O ice and D2O and HDO isolated in H2O ice. Isotopic isolation greatly simplifies the spectra. The observed bands were assigned to hindered rotations around different molecular principal axes. Intermolecular coupling results in remarkable broadening of the librational spectrum, however hindered rotations around the respective axes appear to retain their frequency ordering. Diffusion Monte Carlo investigation was carried out of anharmonic effects. Anharmonic shifts of up to several tens of inverse centimeters are negative for high frequency modes and positive for low frequency ones (with the exception of one HDO frequency in H2O), and affect the ordering of the frequencies. (C) 2003 American Institute of Physics.