We report quantitative pH determinations within the water pool of reverse micelles derived from the surfactant ammonium carboxylate perfluoropolyether (PFPE) formed in supercritical CO2. Toward this end, we use a fluorescent probe, Cl-NERF, that exhibits a pH-sensitive excitation spectrum and locates preferentially within the aqueous core region of the reverse micelle. Sodium 1,4-bis(2-ethylhexyl) sulfosuccinate (commonly know as aerosol-OT or AOT) reverse micelles were formed in liquid n-heptane and used to calibrate the PFPE/CO2 system. We report the effect of CO2 continuous phase pressure and micelle water loading on the pH within the PFPE water pool. We compare the measured water core pH to the value computed assuming CO2 saturation within the PFPE water pool and determine that the PFPE micelle provides a 0.5 pH unit barrier to CO2 partitioning into the micelle water pool.