A reversed-phase HPLC method for the simultaneous determination of the glucosinolate sinigrin and its major degradation product allyl isothiocyanate (AITC) was developed and used for direct analysis of aqueous extracts from Oriental mustard (Brassica juncea L.) related materials (ground and cracked seeds, powders, and bran) and from soil samples. The lowest detection limit was 0.1 microg/mL for both sinigrin and AITC). The developed method was used to trace the degradation of sinigrin to AITC in aqueous extracts. One of the major advantages of this method is the complete estimation of sinigrin content. The simultaneous analysis of both sinigrin and AITC in a single run avoided the underestimation caused by separate analyses.