Room-temperature ionic liquid (RTIL)/water mixtures show acidic properties at around 90 mol % H2O. The RTIL is N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate. The pH of the mixture in the nonequilibrium state is sensitive to the quality of the water type: city water (CW), distilled water (DIS), or ultrapure water (UP). At 90 mol % [H2O](DIS), rhythmic oscillations of pH are observed. The time periodicity is found to be approximately 20 mm. In contrast, 90 mol % [H2O](CW) and [H2O](UP) mixtures show no synchronization in the pH.