Determinations at 22° of the thermodynamic dissociation constant of acetic arid in ordinary water and in deuteriumenriched water (98.0 volume % D20) demonstrate that an ordinary Radiometer glass electrode type G 202A under convenient experimental conditions exhibits the theoretical response to variations in the hydrogen-ion concentration in both solvents in the range between 2 x 10 -2 and 2 x 10 -5 M. The acidity determinations involve standardization and storage of the glass electrode in solutions in HzO and subsequent drying of the glass electrode with mercury before immersing it in the 0.5-ml. deuterium enriched samples. From the electromotive forces observed with a saturated KC1-H20 calomel electrode at 22° the relation between true p(DH) and apparent pH in 98% D2O is derived. p(DH) = apparent pH + 0.44. A possibility of determining the activities, an and aD, separately is considered from the point of view of extrapolating rate data obtained in HzO-DzO mixtures to pure D2O.