In this study organic compounds in dichloromethane extracts of wastewater samples taken at different stages of a mineral oil refinery wastewater treatment plant were analysed by gas chromatography using a mass selective detector. Main constituents of the raw wastewater were n-alkanes, iso-alkanes, cyclic alkanes, aromatic hydrocarbons, and phenols. Also small concentrations of some heterocycles were detected. The data suggest that flotation is a suitable pre-treatment step for removing the major part of alkanes, aromatics and phenols. The flotation protects biological stages against inhibitory effects of e.g. phenols. The activated sludge process removes most of the remaining gas chromatographically detectable organics and only three iso-alkanes, traces of carboxylic acids and – exhibiting the highest concentration – acetic acid 2-(2-butoxyethoxy)-ethyl ester were found in the clarified effluent of the activated sludge process. These detected substances cannot explain the relatively high COD of the sedimentation effluent and it is assumed that the main part of the COD of the biologically treated mineral oil refinery wastewater is represented by humic material formed in the activated sludge process. As in the raw wastewater also volatile aromatics (e.g. toluene, xylenes, ethylbenzene) were detected it is recommended to collect and treat the off-gas of flotation units applied for petrochemical wastewater purification.