Glucosinolates are secondary plant metabolites occurring in Brassicaceae plants. Upon tissue disruption these compounds can be enzymatically hydrolyzed into isothiocyanates. The latter are very reactive and can react with nucleophiles during food processing such as cooking. Here, a novel type of glucosinolate degradation product was identified resulting from the reaction of the isothiocyanates sulforaphane and allyl isothiocyanate with thioglucose during aqueous heat treatment. The cyclic compounds were isolated and their structure elucidated by NMR spectroscopy and high resolution mass spectrometry as 4-hydroxy-3-(4-(methylsulfinyl)butyl)thiazolidine-2-thione and 3-allyl-4-hydroxythiazolidine-2-thione. Based on experiments with isotope-labeled reagents, the determination of the diastereomeric ratio and further reactions, a reaction mechanism was proposed. Finally, the formation of the two 3-alk(en)yl-4-hydroxythiazolidine-2-thiones was quantified in boiled cabbage samples with contents of 92 pmol/g respectively 19 pmol/g fresh weight using standard addition method.