AbstractHydroxy‐dechlorination,\documentclass{article}\usepackage{amsmath}\pagestyle{empty}\begin{document}$ {\rm ZC}_6 {\rm H}_4 {\rm Cl} + ^ \bullet{\rm OH}\xrightarrow{{(9)}}{\rm ZC}_6 {\rm H}_4 {\rm OH} + {\rm Cl}^ \bullet $\end{document}is advanced as a straightforward process enabling measurement of structural effects on rates of addition of\documentclass{article}\pagestyle{empty}\begin{document}$ ^ \bullet {\rm OH} $\end{document}to substituted benzenes. Hydroxyl radicals were generated fromtert‐butyl hydroperoxide. Under the conditions employed (gas phase, 290°C), reaction (9) with chlorobenzene is 0.3 times as fast as H abstraction to giveo/m/p‐\documentclass{article}\pagestyle{empty}\begin{document}$ {\rm ClC}_6 {\rm H}_4 ^ \bullet $\end{document}radicals. Analogousipsosubstitution of methyl was not observed; withp‐chlorotoluene,\documentclass{article}\pagestyle{empty}\begin{document}$ ^ \bullet {\rm OH} $\end{document}forms\documentclass{article}\pagestyle{empty}\begin{document}$ {\rm ClC}_6 {\rm H}_4 {\rm CH}_2 ^ \bullet $\end{document}at least 30 times faster than it gives reaction (9); witho‐ andp‐ClC6H4CF3(2c), both reaction 9 and displacement of CF3to give ClC6H4OH (reaction 12) occur, the latteripsosubstitution beingca.19 times faster witho‐ than withp‐2c. A thermokinetic rationale is given, for what is apparently a steric effect. Site selectivities for reaction (9), expressed as isomer distributiono/m/p, are: 55/25/20 for Z = Cl and 54/31/15 for