The pyrolysis of ethylene has been studied in the temperature range 50G600 "C and the pressure range 15-60 cm. The main products were ethane, propylene, butene, butadiene, and a polymer of molecular weight corresponding to C, or higher. Small amounts of methane, butane, unsaturated C5, unsaturated C6, and benzene were also measured. Of the main products, propylene, butene, and butadiene showed an induction period, as long as several minutes at the lowest temperature. The order with respect to ethylene of ethane, propylene, and butene was close to two and the activation energy of the rates was approxin~ately 40 kcal/~nole. The results have been interpreted in terms of a free radical chain polymerization. It is suggested that the polymer formed is unstable and decomposes to yield the products for which an induction period was observed.