Abstract Problems related to the replacement by deuterium of the hydrogen engaged in H-bonding are discussed. Anomalous isotope effects on the position (νAH/νAD, ν′AH/ν′AD or ν″AH/ν″AD) and the integrated intensity ( A H/ A D or P H/ P D) of the νAH vibration are reviewed. IR spectra of H-bonded complexes with anomalous isotope effects show a broad absorption overlapping of the skeletal vibrations. The separation of these two absorptions is not simple and consequently leads to qualitative values of the isotope ratio. There is no doubt that the isotope frequency ratio for H-bond complexes decreases up to unity, but the increase to > 2 1 2 is less certain. The calculated values of the isotope ratio seem to be overestimated. The relation between the isotope effect and potential function has fundamental significance and requires more precise values of the frequency isotope ratio.