TBC is added as an oxidation inhibitor to 1,3-butadiene for industrial use. For the determination the inhibitor is separated from the butadiene by evaporation. The residue is dissolved in water and an excess of iron(III) chloride is added. The spectrometric measurement of the absorbance of the yellow-coloured complex is carried out at a wavelength of about 425 nm. The method is applicable to butadiene containing more than 50 mg TBC per kilogram.