The production of clean oil by thermal degradation of municipal waste plastics pretreated with supercritical alkaline alcohols has been investigated. When the municipal waste plastics was treated with the alcoholic solutions of NaOH at 300 degrees C, the chlorine and nitrogen atoms present in the waste plastics eluted in the order of methanol > ethanol > 2-propanol > tertiary butanol. In the pretreatment with a methanolic solution of NaOH at 250-300 degrees C for 30 min, these heteroatoms eluted in almost the same yields, 91.4-94.4 and 76.0-76.8%, respectively. However, the yields of these heteroatoms eluted decreased to 59.7 and 71.7%, respectively, in the pretreatment at the subcritical temperature of 225 degrees C. In the pretreatments at 250-300 degrees C for 30 min, the total chlorine content in the resultant oils was 16 ppm and it increased markedly to 85 ppm in the pretreatment at 225 degrees C. The total nitrogen content in the oil increased gradually from 34 to 64 ppm upon temperature decrease from 300 to 225 degrees C at the pretreatment step. An oil, whose total chlorine and nitrogen contents were 16 and 47 ppm, respectively, was thus produced at the yield of 62 wt% via the pretreatment with methanolic solution of NaOH at 250 degrees C. The boiling point distribution, type of hydrocarbons, and physical and physicochemical properties of the oil such as kinematic viscosity and pour point were determined and compared with those of the oils produced in the commercial plants in Japan.