Large changes in the aqueous phase PH of a buffered biphasic water-CO2 system can be readily achieved through manipulation of the temperature and pressure of the system. Using NaOH, PH 'switches' greater than 1.5 pH units were readily obtained simply by fluctuating the CO2 Pressure over a range of 400 bar. The phenomenon of PH 'switches' in the aqueous phase of a biphasic water-CO2 system was monitored experimentally by UV-visible spectroscopy using hydrophilic indicators. Prior to experimentation, a thermodynamic optimization model was developed to maximize these pH 'switches' with changes in temperature and pressure. We also report for the first time an aqueous phase pH within a water-CO2 system above neutrality. (C) 2002 Elsevier Science B.V. All rights reserved.