A combined titration methodology at constant pH and dissolved oxygen concentration to monitor the degradation of organics as carbon dioxide evolution rate (CER) and oxygen uptake rate (OUR) is presented. Equations necessary to assess CER from alkali titration rates and the instrument used to test the technique are described. Experiments were performed on samples of activated sludge from a domestic wastewater treatment plant using glucose and ethanol as substrates. OUR and CER were calculated for both substrates, as well as the respiratory quotient RQ = CER/OUR and biomass to substrate yields coefficients. RQ data for ethanol compared better with corresponding stoichiometric values than those for glucose.