The seed of Limnanthes douglasii, a herb native to Oregon and California, furnished the previously unknown m-methoxybenzyl isothiocyanate, which was also obtained by synthesis. Previous studies showed the plant afforded a pungent, steam-volatile oil that appeared to be an isothiocyanate and gave a crystalline thiourea with ammonia, and the thiourea from its seed had a distinct Rph value. Analyses of the mustard oil and derivatives indicated the isothiocyanate had the empirical formula C₉H₉ONS and contained one methoxyl group. Ultraviolet and infrared spectra suggested it contained methoxyl, phenyl ether, and unconjugated isothiocyanate groups. Permanganate oxidations of the mustard oil and thiourea yielded m-methoxybenzoic acid, confirming the structure as m-methoxybenzyl isothiocyanate. m-Methoxybenzaldehyde was converted to m-methoxybenzylamine via the Leuckart reaction, which was then used to synthesize m-methoxybenzyl isothiocyanate. The natural and synthetic mustard oils were identical.