<jats:title>ABSTRACT</jats:title><jats:p>The individual glucosinolates present in <jats:italic>B. juncea</jats:italic> mustard seed were determined. Sinigrin (allyl glucosinolate) was the major glucosinolate of mustard seed and constituted 92.9% of the total glucosinolates. The effect of CH<jats:sub>3</jats:sub>OH/NH<jats:sub>3</jats:sub>/H<jats:sub>2</jats:sub>O‐hexane treatment on the concentration of glucosinolates was studied. Over 97% of sinigrin originally present in the seed was removed by the above solvent extraction system, mainly in the intact form. Breakdown products of sinigrin were quantified in the treated meal, in the extracted oil and in the polar phase (gums). Aglycon products were allyl cyanide, allyl isothiocyanate and 2,3‐epithiopropane cyanide. Sugar breakdown products were glucose, thioglucose, thioglucose dimer and furfuryl alcohol. Desulfosinigrin was a minor decomposition product and might have acted as an intermediate in the decomposition of sinigrin.