Product branching ratios for the gas-phase reactions of i-butanol, (CH3)2CHCH2OH, with OH radicals (251, 294, and 340 K) and Cl atoms (294 K) were quantified in an environmental chamber study and used to interpret i-butanol site-specific reactivity. i-butyraldehyde, acetone, acetaldehyde, and formaldehyde were observed as major stable end-products in both reaction systems with carbon mass balance indistinguishable from unity. Product branching ratios for OH oxidation were found to be temperature dependent with the α, β, and γ channels changing from 34 ± 6 to 47 ± 1%, 58 ± 6 to 37 ± 9%, and 8 ± 1 to 16 ± 4%, respectively, between 251 and 340 K. Recommended temperature dependent site-specific modified Arrhenius expressions for the OH reaction rate coefficient are (cm3 molecule–1 s–1): kα(T) = 8.64 × 10–18 T1.91 exp(666/T) kβ(T) = 5.15 × 10–19 T2.04 exp(1304/T) kγ(T) = 3.20 × 10–17 T1.78 exp(107/T) kOH(T) = 2.10 × 10–18 T2 exp(–85/T) exp(-61.69/T) where kTotal(T) = kα(T) + kβ(T) + kγ(T) + kOH(T). The expressions were constrained using the product branching ratios measured in this study and previous total phenomenological rate coefficient measurements. The site-specific expressions compare reasonably well with recent theoretical work. It is shown that use of i-butanol would result in acetone as the dominant degradation product under most atmospheric conditions.