The cationic polymerizations of styrene under an electric field have been investigated by using boron trifluoride etherate as a catalyst and binary mixtures of toluene, 1,2-dichloroethane, and nitrobenzene of various compositions as the solvent. The polymerization rate of styrene in dichloroethane was tripled in the presence of a rather low-intensity electric field of 0.25 kV/cm. The field-accelerating effects were found to be nil in pure toluene, increasing with increasing dielectric constant of the mixed solvents and, through the maximum value in pure dichloroethane, decreased with further increase in the dielectric constant. This observation might indicate that the ion pair of the growing chain ends is too “firm” to show the field effects in toluene, whereas the degree of dissociation of ion pairs in nitrobenzene is large to such an extent that the application of the electric field can no longer increase the population of free ions. This interpretation is in qualitative agreement with the prediction of the kinetic scheme presented previously. On the other hand, the degrees of polymerization were not influenced by the electric field.