As a part of the improvement in liquefaction of waste expanded polystyrene (EPS), the pyrolysis of EPS with cu-methylstyrene (MSM) or 2,4-diphenyl-4-methyl-1-pentene (MSD) was carried out at a temperature below 200 degrees C. These additives are expected to possess the merit of a solvent, a chain transfer agent, and a polymerization inhibitor combined. The additional effects of those were compared with that of (R)-(+)-limonene (limonene). To determine the optimal conditions for pyrolysis of EPS, the influence of the following factors on number-average molecular weight ((M) over bar(n)) of EPS was studied:molar ratio of an additive to EPS, temperature of heat treatment, and time of that. When MSM was used as an additive, the (M) over bar(n) of EPS was lowered effectively at [MSM]/[EPS]=1, while in case of MSD, the most efficient reduction in (M) over bar(n) of EPS was attained at [MSD]/[EPS]=0.5. The additional effects were shown maximally at about 160 degrees C and the pyrolysis of EPS proceeded considerably at time of the order of 2 h. MSM was superior to MSD as both a good solvent and an effective chain transfer agent. On the other hand, though limonene acted an excellent solvent for EPS, the chain transfer effect on EPS was less than expected. Through the pyrolysis of monodispersed oligostyrene, it was possible to demonstrate a chain transfer mechanism with addition-fragmentation reaction.