AbstractThe rate of the thermal reaction of ethylene to form cyclobutane has been measured over the temperature range 723°–786°K and at pressures between 300 and 1300 torr. The equilibrium constant for the system\documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}$${\rm 2C}_{\rm 2} {\rm H}_{\rm 4}\mathop {\leftrightharpoons}\limits_{kf}^{kr} c - {\rm C}_{\rm 4} {\rm H}_{\rm 8}$$\end{document}was calculated both from the initial rate data and from measurements of the equilibrium concentration of cyclobutane. Agreement with the reported thermodynamic quantities for cyclobutane was satisfactory. The initial rate data gave the following epxression forkf:\documentclass{article}\pagestyle{empty}\begin{document}$${\rm log }k_f {\rm (1}{\rm .mole}^{ - {\rm 1}} {\rm sec}^{ - {\rm 1}} {\rm)} = {\rm 7}{\rm .84} - \frac{{43800}}{{2.3RT}}$$\end{document}while the measurements of the equilibrium concentration of cyclobutane gave the expression forK,\documentclass{article}\pagestyle{empty}\begin{document}$${\rm log }K{\rm (atm}^{ - {\rm 1}} {\rm)} = - \frac{{45.4}}{{2.3R}} + \frac{{20700}}{{2.3RT}}$$\end{docume