The gas chromatographic (GC) determination of trace amounts of j?-methylmercaptopropionaldehyde (methional) in the free form using a flame photometric detector (FPD) was investigated_ The GC conditions were as follows: stationary phase, 6 y0 DEGS; support, Chromosorb W HP (SO-100 mesh); glass column, 2 m x 3 mm I-D_, column temperature (progranum -ng), holding for 1 min at lOOC, heating the column oven at a rate of lO"C/min from 100 to 19O'C, maintaining this temperature for 20 min and then cooling to the initial temperature; carrier gas, nitrogen, flowrate, 60 mlfmin; llow-rates of hydrogen and air for the FPD, each 40 ml/min. The minimum detectable amounts for the methional were CQ. 0.3 ng; the repeatabilities of retention times and peak areas (measured as integrator counts) at I-ng level of the compound were less than ca. 1.5 o/o and ca. 15 %_ The percentage recovery in the concentration method by bubbling the nitrogen carrier gas (0.2 l/min, at 28°C) through the standard solution of methional (at 1 ng per ,~l ethanol) from 10 to 0.5 ml was CQ. 72%; however, the percentage recovery of the standard solution (at 10 ng/pl ethanol) after 10 1 of the laboratory air had been passed through the impinger containing the standard solution at a rate of OS l/min was 97.5 %_ The method was applied to the determination of mcthional in air over waste water from a corn starch factory.