Nonenal synthesis from 9(S)-hydroperoxy-linoleic acid by hydroperoxide-lyase-catalysed cleavage was followed by a precise headspace analysis with GC-MS. Enzyme activity was extracted from plant leaves and fruits. 3Z-nonenal was the main product and a small amount of 2E-isomer was detected since the reaction-analysis procedure used avoided supplementary volatile fraction transformation. The substrate transformation and volatile purge were carried out successively and without delay or any contact with air.