In gaseous reactions important for the understanding of the upper atmosphere ${\mathrm{H}}_2\mathrm{O}$ and $\stackrel{.}{\mathrm{O}}$ react bimolecularly to form two $\stackrel{.}{\mathrm{OH}}$ radicals. $\mathrm{\Delta H}$ for this reaction is $72 \mathrm{kJ}$ at $500 \mathrm{K}$ and ${\mathrm{E}}_{\mathrm{a}}$ is $77 \mathrm{kJ}{ \mathrm{mol}}^{-1}$, then ${\mathrm{E}}_{\mathrm{a}}$ for the bimolecular recombination of two $\stackrel{.}{\mathrm{OH}}$ radicals to form ${\mathrm{H}}_2\mathrm{O}$ and $\stackrel{.}{\mathrm{O}}$ is: