The active TBC in a test portion is extracted by mixing with a solution containing m-nitrophpenol (as the internal standard) and evaporating the butadiene. The TBC and m-nitrophenol are separated by high-performance liquid chromatography and detected by UV: The peak areas or peak heights are measured and the TBC content is determined by using a calibration graph. The polymerization inhibitor TBC can also be determined by a spectrometric method (ISO 6684) but that method is not able to differentiate between the active inhibitor and its oxidized, inactive form. The present method specifically measures the concentration of the active form of TBC. The method is applicable to butadiene having TBC contents in the range 0 to 250 mg/kg.