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Two nonvolatile flavor precursors occurring in Allium vegetables, S-propyl-L-cysteine and its sulfoxide, were heated in closed model systems at different temperatures (from 80 to 200 degrees C) in the presence of variable amounts of water (0-98%) for 1-60 min. It seems to be indisputable that thermally generated breakdown products of both S-propyl-L-cysteine and particularly S-propyl-L-cysteine sulfoxide can significantly participate in the aroma formation of culinary processed Alliumvegetables. Dipropyl disulfide, dipropyl trisulfide, propylthiol, and dipropyl thiosulfonate were identified as the predominant volatile compounds generated by thermal degradation of S-propylcysteine sulfoxide. Dipropyl disulfide and 2-(propylthio)ethylamine were the major breakdown products formed from S-propylcysteine. Substantial amounts of various alkyl- and alkylthio-substituted pyridines were also generated from both S-propylcysteine and its sulfoxide.