AbstractRecent investigations into the processing of oriental mustard seed for production of livestock feed and of lipid‐containing erucic acid have stimulated interest in the composition of the by‐product essential oil. The development of a method for analysis of this oil by gas‐liquid chromatography has led to the demonstration of an equilibrium reaction between the two main constituents. A similarity in composition and response to heat is shown between the natural essential oil and oils prepared synthetically. These observed similarities should lead to a better understanding of the enzymatic reaction responsible for release of this oil from the seed.