Mixtures of ethane and ethylene have been pyrolyzed in the temperature range 563–600 °C and at pressures from 30–60 cm. The products were similar to those obtained from the pyrolysis of ethylene by itself, described m Part I, with a marked increase in the yields of the saturated products. The initial rates of product formation and the dependence of these rates on the concentration of ethane suggest that the initiation step is the same as that proposed in the pyrolysis of ethylene alone, viz.[Formula: see text]and that the reaction[Formula: see text]is not an important source of radicals. A simplified mechanism is outlined to account for the main effects of ethane on the free radical chain polymerization.