Summary 1. Quantitative isotopic exchange reactions have been carried out with free hexoses, their methylglycosides and other derivatives in concentrations of 11-30 and 80- 96% DzO. It is shown that consistent results can be obtained only if the equilibrium constant K for the expression ROH+ HOD e ROD+ HOH is considered explicitly. 3. A common K, numerically 0.65, is used for the calculation of the exchange number of twentyone ...