Recently, we have demonstrated the possible production of clean fuel oils from the municipal waste plastics through hydrothermal pretreatment with aqueous alkaline solutions followed by the thermal liquefaction (Akimoto et al., 2004). In the present work, behaviors of the nitrogen atoms present in nylon, polyurethane and ABS and their interaction with other plastic components during these pretreatment and liquefaction steps have been investigated. When the respective mixtures of the municipal waste plastics with these nitrogen-containing plastics were thermally liquefied at 450 degrees C, the total nitrogen content in oil followed the order of A BS > nylon > polyurethane. In contrast, the total nitrogen content in the carbonaceous residue followed the order of polyurethane > nylon > ABS. Upon hydrothermal pretreatment with 0.2 N NaOH at 300 degrees C, the nitrogen atoms present in these plastic mixtures were eluted in the order of polyurethane > nylon > ABS. In contrast to the nitrogen atoms in polyurethane, thus, the nitrogen atoms in ABS were the most difficult to eliminate at the hydrothermal pretreatment step although they were most likely to remain in oil at the thermal liquefaction step. In contrast to nylon and polyurethane, however, ABS did not always hamper the liquefaction of other plastic components.