Abstract: The invention provides a method and apparatus for providing periodical measurements of the real-time readily biodegradable chemical oxygen demand (RBCOD) of a wastewater stream. The apparatus includes a bio-reactor having an inlet for accepting a feed from the stream and an overflow outlet, whereby a sample volume can be maintained within the reactor. A feed rate is chosen to ensure an hydraulic retention time (HRT) in the reactor sufficient for substantially complete oxidation of the readily biodegradable compounds in the feed. Air is periodically passed through the sample volume for set periods and the oxygen up-take (OUR) is determined by measuring the dissolved oxygen content via a probe during the air-off periods. An RBCOD value is then calculated for each OUR value. The invention may be used to monitor the performance of wastewater treatment plants.