Carbon dioxide (CO(2)) online off-gas monitoring is useful to detect changes in biological activity for activated sludge systems especially under limited oxygen conditions like under simultaneous nitrification-denitrification (SND) where respirometric measurements are not applicable. So far, the influence of the bicarbonate system on the liquid-gas transfer of CO(2) prevented the wider use of off-gas CO(2) for monitoring purposes in wastewater treatment. The objective of the paper is to demonstrate a practical method to correct measured off-gas CO(2) as an indicator of biological activity by taking into account pH shifts (resulting in CO(2) release or retention) and changes in influent alkalinity. The simple model is based on the physicochemical system of the bicarbonate/CO(2) equilibrium and the liquid-gas mass transfer for aerated systems. Standard on-line measurements (pH, temperature, flow rates) and periodical alkalinity titration serve as input data to estimate the influence of the carbonate system on the CO(2) off-gas concentrations measured on-line. For a particular plant the CO(2) mass transfer coefficients are derived from measurements compared to the theoretical calculation from oxygen mass transfer. The model estimates the biological carbon dioxide production rate (CPR; heterotrophic activity) by the correction of the measured carbon dioxide transfer rate (CTR; C-flux by the off-gas) with the calculated inorganic carbon dioxide transfer rate (r(F)) considering bicarbonate consumption (autotrophic activity).