This paper reports the development of a novel detection method, based on the coupling of a biosensor measuring device and a flow-injection system, using the enzyme formaldehyde dehydrogenase and a Os(bpy)(2)-poly(vinylpyridine) (POs-EA) chemically modified screen-printed electrode. The sensor can detect 30 ng ml(-1) of formaldehyde in aqueous solution (corresponding to sub-ppb atmospheric concentrations of formaldehyde). The sensor is selective, inexpensive, stable over several days, and disposable, as well as simple to manufacture and operate. The system described here can easily be adapted to other substrates using their corresponding dehydrogenases.