The gas phase OH hydrogen abstraction reaction from isoleucine is studied using Unrestricted Density Functional Theory (BHandHLYP) calculations and the 6-311G(d,p) basis sets. The structures of the different stationary points are discussed. Ring-like structures are found for all the transition states. Reaction profiles are modeled including the formation of pre-reactive complexes, and negative net activation energy is obtained for the overall reaction. A complex mechanism is proposed, and the rate coefficients are calculated using Transition State Theory over the temperature range 250–350 K. The rate coefficients are proposed for the first time and it was found that in gas phase the hydrogen abstraction occurs mainly from the beta site. The following expressions, in l mol−1 s−1, are obtained for the alpha and beta H-abstraction channels, and for the overall temperature dependent rate constants, respectively: and