2,4,6-Trichlorophenylhydrazine was tested as a reagent for carbonyl compounds. As little as 0.1 of the 2,4,6-trichlorophenylhydrazones (2,4,6-TCPH) could be measured with an electron capture detector, so this reagent should be useful in measuring the carbonyl compounds in oxidized fats at levels near their flavor thresholds. Mixtures of 2,4,6-TCPHs were separated by thin layer chromatography. Alkan-2-one-2,4,6-TCPHs were separated from aldehyde-2,4,6-TCPHs on alumina plates. The alkanal, alk-2-enal and alk-2,4-dienal-2,4,6-TCPHs were separated from each other either on silica gel plates or silica gel-silver ion plates. The derivatives within each carbonyl class were separated by chain length on chromatography media impregnated with phenoxyethanol. The 2,4,6-TCPHs eluted from thin layer plates were determined with an electron capture detector after gas chromatography on a 30 cm column of freeze-dried detergent base coated with a silicone oil.