A general and simple equation is presented to account for the effect of CO2 absorption and the dissociation of carbonic acid in liquid on the measurement of the CO2 evolution rate (QCO(2)) of both anaerobic and aerobic continuous cultures. For aerobic cultures the same equation applies to the measurement of the respiratory quotient (RQ). The deviation of QCO(2) and RQ, calculated from gas-phase measurements, from their true values can be assessed with two parameters : one accounts for the influence of pH, resulting from dissociation of carbonic acid, the other for the influence of operating conditions. Plots are given to show the influences of culture and operating conditions; they may be used as a guideline for choosing proper operating conditions for a reliable measurement of CO2 evolution rate and RQ value.