AbstractRecently, we have discovered the unusual inverse temperature dependence of the kinetic isotope effect (KIE) for the OH + c‐C6H12/c‐C6D12 reaction in water. Temperature increase causes a KIE increase; this is valid for both the Fenton system (I) and the HOONOH2O system (II) (a new source of OH radicals in alkane reactions), whereas in the gas phase KIE decreases with increasing temperature. Results of these studies are considered. The KIE temperature dependences for both reactions in solution, (kH/kD)w = (2.65 ± 0.15) exp[(−2.0 ± 0.1)/RT], ED−EH in kJ mol−1, and gas‐phase reactions, (kH/kD)g = (1.2 ± 0.1) exp[(+2.0 ± 0.1)/RT], differ significantly. The anomalous temperature behavior of KIE in water is described by a cage effect model at the step of the reactant encounter, which was proposed earlier by Rudakov. The model can be represented in a concise form: RH + OH \documentclass{article}\usepackage{amssymb}\begin{document}\pagestyle{empty}$\begin{array}{c}{\scriptstyle 1}\\ \rightleftarrows \\ {\scriptstyle -1}\end{array}$\end{document} 〈RH··OH〉* \documentclass{article}\usepackage{amssymb}\begin{document}\pagestyle{empty}$\begin{array}{c}{\scriptstyle 2}\\ \rightarrow\end{array}$\end{document} R• + H2O, where 〈RH··OH〉*is the oriented encounter complex in the water cage. It is suggested that elementary acts of H‐abstraction (step 2) are identical for both gas phase and solution. It allows calculation of the competition parameter between steps 2 and –1 (the alkane leaves the cage via diffusion), (k−1/k2H) = [(kH/kD)g− 1]/[(kH/kD)g− (kH/kD)w], by using the experimental KIEs. A temperature‐dependence average for systems I and II was found as (k−1/k2H) = (245 ± 35) exp[(18.5 ± 0.5)/RT]. The hypothetical dependence of (kH/kD)w on T was calculated within the overall temperature range (by extrapolations outside the experimental area, 278–328 K), and its connection with that of (kH/kD)g on T is considered. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 557–562, 2011