In previous work on the thermal degradation of polystyrene of average molecular weight of 230,000, carried out in a vacuum in the temperature range 318 to 348 °C, the rate curves exhibited distinct maximums when percentage loss of sample per minute was plotted as a function of percentage volatilization. These maximums correspond to a volatilization of about 35 to 45 percent and tend to flatten with decreased temperature of pyrolysis. A similar study of rates of thermal degradation at 307.5 °C has now been made on two groups of polymers: (1) low molecular weight, 24,000; 51,000; and 66,000; and (2) high molecular weight, 2,000,000; 2,250,000; and 5,000,000. Whole polymers and also fractions of narrow-range molecular weight were used. The rate curves for the 24,000 and 51,000 molecular-weight samples exhibit very high initial rates, but no maximums; the 66,000 sample showed a maximum at 45 percent volatilization. The rate curves for the 2,000,000, 2,250,000, and 5,000,000 molecular-weight samples exhibit a gradual rise up to about 25 percent volatilization; then, instead of forming maximums, they follow plateaus to about 50 percent volatilization. These plateaus are indicative of a zero-order reaction in the range that they cover, and the rates corresponding to them fit well on the Arrhenius activation-energy curve obtained previously for polystyrene samples at higher temperatures.