The pH oscillations related to the protonation/deprotonation process in a room temperature ionic liquid (RTIL)-water system were observed at fixed temperatures. The RTIL was hydrophilic N, N-diethyl-N- methyl-N-2-methoxyethyl ammonium tetrafluoroborate ([DEME][BF4]). The rhythmic oscillations of pH were observed only at a water concentration of approximately 90 mol%, where the equilibrated pH value was approximately 3. In contrast, in the water-poor region from 6 to 10 mol% H2O, the pH was almost constant at approximately 8 within the investigated timeframe. The acid-base properties of the [DEME][BF4]-water mixture are related to particular aggregations in the liquid phase. (C) 2014 Elsevier B.V. All rights reserved.