The crystallization behavior of pure vitreous selenium and selenium doped with various concentrations of arsenic, chlorine, bromine, and iodine was investigated by a dilatometric method, with particular emphasis on constancy of the thermal pretreatment of samples. The energy of activation for the crystallization of vitreous selenium was found to be 25 kcal/mole. Arsenic had a very pronounced retarding effect at the 100 ppm level. Chlorine and iodine showed a complex behavior, whereas bromine showed a moderate accelerating effect on the rate of crystallization of selenium.