The discovery of the hydrogen isotope, deuterium, was recognised as one of the most important events in the history of physical chemistry in the last twenty years. The comparatively large differences in the physical properties of the hydrogen isotopes and their compounds led immediately to the use of deuterium as an indicator and also as a tool -rith which to investigate the mechanisms of reactions involving hydrogen and its compounds. The commercial preparation of deuterium oxide ('heavy water') and its availability at a comparatively low price has enabled chemists to use this isotopic water in place of ordinary water. In the field of homogeneous catalysis this has been particularly advantageous and as the following work deals with homogeneous catalysis in isotopic water solutions I append a short summary of the work hitherto done and the relevant ideas held at present.