The yield of polymer formed by low-temperature irradiation of styrene in methylene dichloride has been measured at various concentrations of monomer and at different intensities. Corresponding yields were also determined when polymerization occurred in the presence of Aerosil (fine silica powder). Analysis of the results by means of a Mayo type of plot shows that the increased yield with Aerosil is due to a reduction in the termination step. Chain transfer during propagation is found to explain the relatively small difference in measured molecular weights. Over much of the range studied the intensity exponent is about 1 without and 0.5 with Aerosil. The former agrees with the usual kinetics involving a termination by the original gegenion, whereas the latter requires termination by a species produced in a separate ionizing event.