A membrane inlet system to a mass spectrometer (MIMS) allows us to monitor the abundance of18O in CO2during the exchange of18O between HCO3−, dissolved CO2and H2O that occurs after dissolving18O-labelled NaHCO3in aqueous solution. Using a mathematical model we quantitate intracellular carbonic anhydrase activity and membrane permeabilities for HCO3−, CO2and H2O of isolated cells or intact epithelia added to the solution. For these calculations it was necessary to determine (a) the relation between MS signal and [C18O16O], (b) MIMS response kinetics and (c) CO2leakage during the experiment. The three parameters were determined by stepwise addition of HCl to HCO3−solution: (a) was found to be linear, (b) response times were 7.5±2s (10°C), 4.5±1s (20°C), 3.5±0.6s (37°C), and (c) CO2leakage was < 1‰ s−1.