Photodegradation oC coumaphos and azinphos-methyl in lhe air, in oxygen, and in nitrogen atmosphere (quartz reactor eyuipped with six 18-W lamps with the maximum emission intensity al 310 nm) was sludied. The faslest reaclion occurred with coumaphos in nitrogen almosphere (complete decomposilion in 10 min), where the formation of a new compound was also detected by high-performance liquid chromatography (HPLC). In lhe case of azinphos-methyl, no additional signals in HPLC chromalogram were observed. The rearrangement mechanism, involving a triplet-state activation complex and the formation of an acetylcholinesterase (AChE)-inhibiting oxo analog of coumaphos was verihed and con[irmed by laser flash photolysis. The suitability of bioanalytical flow-injection analysis (FIA) systems, based on AChE inhibition and speclrophotometric or thermal lens spectrometric deteclion for rapid and sensitive screening in food qualily control was demonstrated. Owing to the high sensitivity of thermal lens speclrometry (TLS), several steps in sample preparation can be avoided (preconcentration, purilication, isolation) and incubation times reduced. High sample throughpuls (10 h-I) are, however, also achievable for medium toxic oxo organophosphorus (OP) pesticides using spectrophotomelric detection providing limits of detection (LODs) al lhe level of 10 ppb malaoxon equivalents, which is still about 50 times below maximal residue levels. Testing of the melhod for practical application on a set of 60 samples gave no false negalive results and a level of 1.7 % false positives.