I HAVE carried out determinations of D2O content in water using the vapour pressure osmometer `Mechrolab, Model 301 A', designed to perform molecular-weight determinations. Briefly, the principle of this instrument is as follows1,2: two thermistors, charged with a drop of solvent and arranged in a d.c. Wheatstone bridge, are at the same temperature in an ambient atmosphere saturated with the vapour of the selected solvent. The bridge is equilibrated, and a drop of a solution of the same solvent is placed on one of the two thermistors (called the sample thermistor). Solvent condenses on this drop of solution until vapour pressure equilibrium is approached, and the temperature of the thermistor increases. Its resistance variation depends on the concentration of the solution and it is determined by rebalancing the bridge.