BACKGROUND: A biochemical oxygen demand (BOD) sensor, based on an immobilized Pseudomonas syringae in highly porous micro-cellular polymer (MCP) in combination with a dissolved oxygen electrode, has been developed for the analysis of biodegradable organic compounds in aqueous samples. Microorganisms were immobilized in a molded MCP disk and a wastewater sample was injected into the biocomposite disk by a flow injection system. Dissolved oxygen (DO) changes as a measure of soluble BOD was read with a DO probe placed into a flow cell carrying biocatalytically activated disk. RESULTS: Optimal response of the MCP BOD sensor was obtained at pH 6.8 and 25 degrees C with a typical response time of 3-5 min for a 2 mm thick molded polymeric disk. The sensor showed detection linearity over the range 5-100 mg L-1 BOD5 (r(2) > 0.99) at a flow rate of 0.6 mL min(-1). The repeatability and reproducibility of the sensor response were found to be 3.08% and 7.77%, respectively. BOD values produced with this biosensor for various municipal and industrial wastewaters correlated well with those determined by the conventional 5-day BOD test. CONCLUSION: This new biosensor was different from present amperometric BOD biosensor configurations in which the biocatalyst (microbial/enzymatic) is placed between cellulose and Teflon membranes installed on a DO probe. The use of a molded MCP disk coniainng microbial activity offers better stability and lifetime for commercial use in environmental monitoring. (c) 2008 Society of Chemical Industry