A liquid or a vaporized gaseous test portion is injected into a gas chromatography column. After passage of the test portion through this column the oligomers are separated. The detection is performed by flame ionization. The peaks obtained are compared with those derived from an external standard. The oligomers which may be present are: 4-vinyl cyclohexene, 1,3-cyclooctadiene, 1,5-cyclooctadiene. The method is applicable to butadiene having oligomer contents greater than 10 mg/kg.