AbstractRelative rate constants for the gas‐phase reactions of OH radicals with a series of bi‐ and tricyclic alkanes have been determined at 299 ± 2 K, using methyl nitrite photolysis in air as a source of OH radicals. Using a rate constant for the reaction of OH radicals with cyclohexane of 7.57 × 10−12cm3/molec·s, the rate constants obtained are (× 1012cm3/molec·s): bicyclo[2.2.1]heptane, 5.53 ± 0.15; bicyclo[2.2.2]octane, 14.8 ± 1.0; bicyclo[3.3.0]octane, 11.1 ± 0.6;cis‐bicyclo[4.3.0]nonane, 17.3 ± 1.3;trans‐bicyclo[4.3.0]nonane, 17.8 ± 1.3;cis‐bicyclo[4.4.0]decane, 20.1 ± 1.4;trans‐bicyclo[4.4.0]decane, 20.6 ± 1.2; tricyclo[5.2.1.02,6]decane, 11.4 ± 0.4; and tricyclo[3.3.1.13,7]decane, 23.2 ± 2.1. These data show that overall ring strain energies of ≲4–5 kcal mol−1have no significant effect on the rate constants, but that larger ring strain results in the rate constants being decreased, relative to those expected for the strain‐free molecules, by ratios which increase approximately exponen