AbstractMonitoring CO2 production in systems, where pH is changing with time is hampered by the chemical behavior and pH‐dependent volatility of this compound. In this article, we present the first method where the concentration and production rate of dissolved CO2 can be monitored directly, continuously, and quantitatively under conditions where pH changes rapidly (≈2 units in 15 min). The method corrects membrane inlet mass spectrometry (MIMS) measurements of CO2 for pH dependency using on‐line pH analysis and an experimentally established calibration model. It is valid within the pH range of 3.5 to 7, despite pH‐dependent calibration constants that vary in a non‐linear fashion with more than a factor of 3 in this interval. The method made it possible to determine the carbon dioxide production during Lactococcus lactis fermentations, where pH drops up to 3 units during the fermentation. The accuracy was approximately 5%. We used the method to investigate the effect of initial extracellular pH on carbon dioxide production during anarobic glucose fermentation by non‐growing Lactocoocus lactis and demonstrated that the carbon dioxide production rate increases considerably, when the initial pH was increased from 6 to 6.8. © 2005 Wiley Periodicals, Inc.