It was determined by dilatometry that the addition of certain natural materials, i.e., gilsonite, the asphaltene and resin fractions derived therefrom, the Wilmington crude oil, to a bulk styrene polymerization at 86.6°C. catalyzed by benzoyl peroxide modified the rate of reaction. At higher concentrations a period of inhibition was observed followed by retarded rates of polymerization. At lower concentrations an initial acceleration occurred, followed by retarded rates of polymerization. The reaction media with smaller amounts of gilsonite or with the asphaltene fraction became appreciably more viscous with time and in extreme cases changed to a gel; under the same conditions the control system or runs with larger amounts of additives underwent much lesser changes in viscosity. The effect of addition of a metal porphyrin, nickel protoporphyrin IX, was also studied since this type of compound is present in these natural materials. No modification in the polymerization rate was observed. Plots of retarded polymerization rates, observed after initial inhibition or acceleration, versus initial concentration of the several natural additives provided straight lines similar to the relationship found previously for certain synthetic additives. It is proposed that inhibition and acceleration involve natural free radicals in the additives. Gel formation is assumed to result from crosslinking of the polystyrene chains occurring only when natural free radicals and those modifier molecules responsible for retardation are largely spent.