In Brassica vegetables, heating processes lead to thermally induced degradation of glucosinolates (GSLs), resulting in the formation of nitriles and isothiocyanates (ITCs). To date, the mechanism is not yet satisfyingly elucidated. Thermally induced degradation of the model GSL sinigrin was studied in dry as well as aqueous medium at different pH values and temperatures. The influence of the presence of iron ions and plant matrix (broccoli sprouts powder) on the degradation was studied as well. Next to the degradation of the GSL, the formation of nitrile and ITC and the release of sugar derivatives were investigated. Because d-glucose and ITC are main thermal breakdown products under aqueous conditions, hydrolysis seems to be the initial step in the degradation pathway during cooking. In contrast, under dry conditions, the desulfo-sinigrin was identified as a main intermediary thermal breakdown product for the first time. Further, degradation of the desulfo-GSL results in the release of d-thioglucose and the corresponding nitrile. Iron(II) ions and plant matrix influence the thermal stability of the GSL and favor the formation of nitriles.