A new hybrid respirometer is used to measure the oxygen uptake rate (OUR) of COD aerobic biodegradation process. Parameter identification and estimation of carbon oxidation process of an Activated Sludge Model are studied by combining the OUR data and mathematical fitting of curves. Coefficients of variation (CVs) for the three combined parameters estimated are below 13% for activated sludge from a laboratory and below 8% for activated sludge from a wastewater treatment plant. For single parameter estimation, the CVs are below 25% and below 10%, respectively. High measurement frequency and precision of the hybrid respirometer can improve the precision of parameter estimation. Initial concentrations of the substrate and the activated sludge of the respirometric experiment are key conditions that must be optimized to obtain highprecision parameter estimation due to their effect on OUR curve characteristics.