ABSTRACT A technique for the on-line estimation of oxygen uptake rate (OUR), using readily available sensors and a process control computer, is presented. Data from a full-scale, instrumented wastewater treatment plant is used to evaluate the technique. A comparison between the measured and estimated OUR shows a reasonable agreement indicating that the estimated OUR can be used for monitoring and control of the activated sludge process. The specific oxygen uptake rate (SCOUR), an index of sludge activity, can also be estimated by coupling the estimate of OUR with measurement of the mixed liquor suspended solids (MLSS).