AbstractVinylacetylene and 2‐butyne mixtures were pyrolyzed at 350–450°C in the absence and presence of O2and NO. The major product of the reaction is a polymer, but o‐xylene is also produced and was studied as the species of interest. The C8H10formation rate is first‐order in C4H4and C4H6. The rate coefficient is best fitted by\documentclass{article}\pagestyle{empty}\begin{document}$$\log {\rm [}k{\rm (C}_{\rm 8} {\rm H}_{{\rm 10}}),{\rm M}^{{\rm -1}}\, {\rm s}^{{\rm - 1}}] = (4.33 \pm 0.58) - (80.1 \pm 7.4)/2.3RT$$\end{document}though it is not inconsistent with\documentclass{article}\pagestyle{empty}\begin{document}$$\log {\rm [}k{\rm (C}_{\rm 8} {\rm H}_{{\rm 10}}),{\rm M}^{{\rm - 1}}\, {\rm s}^{{\rm - 1}}] = (7.93) - (125.9)/2.3RT$$\end{document}whereRis the ideal gas constant in kJ/mol‐K. O‐xylene formation occurs by two processes: a concerted molecular mechanism (⋍67%) and a singlet diradical me