Viable operating conditions were identified experimentally for maximizing the production of high-value products such as ethylene, propylene, styrene, and benzene, from the ultrapyrolysis of waste plastics. Using both a batch microreactor and a pilot-plant-sized reactor, the key operating variables considered were pyrolysis temperature, product reaction time, and quench time. In the microreactor experiments, polystyrene (PS), a significant component of waste plastics, was pyrolyzed at temperatures ranging from 800 to 965 degrees C, with total reaction times ranging from 500 to 1000 ms. At a temperature of 965 degrees C and 500 ms, the yields of styrene plus benzene were greater than 95 wt%. In the pilot-plant experiments, our recently patented internally circulating fluidized bed (ICFB) reactor (Milne et al., U.S, Patent No. 5,370,789, 1994b) was used to ultrapyrolyze low-density polyethylene (LDPE) in addition to LDPE (5% by weight)/heavy oil mixtures at a residence time of 600 ms. Both experiments produced light olefin yields greater than 55 wt % at temperatures above 830 degrees C.