The kinetics of the formation of butadiene polymer film has been investigated on a surface exposed to 250-e.v. electrons. In the presence of butadiene vapors at pressures between 3 × 10-4 and 10-6 torr, the rate of growth of the transparent film insoluble in benzene, acetone, CCl4, or CS2, was found proportional to the square root of the current density. The rate increased with increasing pressure, becoming independent at higher values of pressure. The kinetics has been interpreted to indicate that after the first layer of polymer has formed, subsequent layers grow by interaction of adsorbed monomer gas with active species, probably positive ions in the polymer The measured lifetime of the active species is about 3 min. The apparent activation energy is -6 ± 2 kcal./mole.