The production of clean fuel oil from the municipal waste plastics without any simultaneous formation of heteroatom-containing organic compounds was investigated. When non-pretreated municipal waste plastics was thermally degraded at 450degreesC, the oil whose total chlorine and nitrogen contents were 3960 and 743 ppm, respectively, was produced at the per cent yield of 53.5 wt%. When the municipal waste plastics was pretreated with 0.2 N NaOH at 300degreesC for 60 min, however, 98% of the chlorines and 82% of the nitrogens originally present in the waste plastics were eluted. The total chlorine and nitrogen contents in the resultant oil decreased dramatically to 22 and 29 ppm, respectively, and at the same time the yield of oil increased to 63.9 wt%. The total chlorine content in oil further decreased to 11 ppm when 0.4 N Ca(OH)(2) was used instead of 0.2 N NaOH. The oil produced from non-pretreated municipal waste plastics contained such heteroatom- containing organic compounds as benzoic acid, phthalic acids and e-caprolactam, and their combined content was as high as 1.68 wt%. In contrast, only 24 ppm of epsilon-caprolactam was found in the oil produced via the hydrothermal pretreatment with 0.4 N Ca(OH)(2) at 300degreesC. The correlations of the contents of chlorines, nitrogens and ammonium ions in oil with those of residual chlorines and nitrogens in the pretreated municipal waste plastics were obtained. The boiling point distribution, type of hydrocarbons, and physical and physicochemical properties such as kinetic viscosity were also analyzed for the oils obtained.