To obtain low polymeric polystyrene (PS), pyrolysis of high polymeric PS in solution was studied in the temperature range from 290 to 400 degrees C by using additives or acid catalysts. The low polymeric PS targeted here was that with average molecular weight of 10(4). When the feed PS was pyrolyzed in tetralin by adding sulfur or diphenyl disulfide, the molecular weight of PS decreased greatly, even at lower temperatures, and the desired low polymeric PS was formed in a relatively large amount at the temperatures below 350 degrees C. The degradation behavior was able to be explained in terms of a random polymer chain scission mechanism initiated by sulfur radicals formed from the additives.