Isotope dilution mass spectrometry was developed for the determination of composition of paraffins, olefins, naphthenes and aromatics in distilled oil produced from the pyrolysis reaction of mixed waste plastics using labeled hydrocarbon internal standards including octane-d(18), dodecane-d(26), hexadecane-d(34), benzene-d(6), toluene-d(8), ethylbenzene-d(10), 1,3,5-trimethylbenzene-d(12) and naphthalene-d(8). This technique made it possible to thoroughly quantify more than three hundred peaks in plastic-derived pyrolysis oil, classify pyrolysis oil into four hydrocarbon groups of paraffin, olefin, naphthene and aromatic, and determine the weight percent of each hydrocarbon group simultaneously. Compared with commercially available petroleum oil, distilled plastic-derived pyrolysis oil contained much more aromatics amounting to 60-82 wt% of whole hydrocarbons. Toluene (C7-benzene) and trimethylbenzenes (C9-benzenes) were the predominant species amounting to 40-50% of whole hydrocarbons in pyrolysis oil with a gasoline range boiling point and 25-35% of whole hydrocarbons in pyrolysis oil with a diesel range boiling point, respectively. (C) 2002 Elsevier Science Ltd. All rights reserved.