The phase behavior of polystyrene (PS) degradation in toluene was investigated in a visual high-pressure reactor. The system of PS and toluene was heated from room temperature and atmospheric pressure to 350 degrees C and 5.0 MPa. It was observed that during this process, PS dissolved quickly, and then, the system appeared to be a homogeneous phase, and PS degradation occurred in this system. With another high-pressure batch reactor, PS degradation was studied in the temperature range 275-360 degrees C and the pressure ranges 4.5-10.5 MPa. The drop-in molecular weights in the temperature ranges and the amount of PS converted into volatile products were observed. Moreover, the effects of the temperature, pressure, and ratio of toluene to PS on the rate of PS degradation are discussed. The kinetic studies showed that the rate of PS degradation in supercritical toluene increased substantially because the degradation occurred in a homogeneous phase. (c) 2008 Wiley Periodicals, Inc.