The reaction of ＇DNBP＇ (2,4-dinitro-6-sec-butylphenol) with toluene, ethylbenzene or styrene and tert-butylperoxide at 110 degrees C leads to 7-sec-butyl-5-nitro-2-phenylbenzoxazole This benzoxazole was also detected in the residues of an industrial styrene fractionation still in which DNBP was routinely used as the polymerisation retarder. The formation of this product from either ethyl benzene or from styrene involves a carbon-carbon bond cleavage step.