AbstractThe initial rates of formation of the major products in the thermal reactions of ethylene at temperatures in the neighborhood of 800 K have been measured in the presence and absence of the additives neopentane and ethane. It has been shown that in the absence of the additive the main initiation process iswhile in the presence of neopentane and ethane the following additional initiation processes occur:From the ratios of the rates of formation of the major products in the presence and absence of the additive the ratioskN/k1andkE/k1were measured over the temperature range of 750–820 K. Taking values from the literature forkNandkE, the following value was obtained fork1:\documentclass{article}\pagestyle{empty}\begin{document}$$ \log k_1 ({\rm L}/{\rm mol} \cdot {\rm s}) = 11.27 \pm 0.6 - \frac{{64,200 \pm 2000}}{{2.3RT}} $$\end{document}Previous results using butene‐1 as additive were rexamined and shown to be consistent with this measurement. From this measurement the following values were derived: ΔHf(C2H3) = 63.4 ± 2 kcal/mol andD(C2H3H) = 103 k