The OH initiated oxidation of isoprene is important in tropospheric chemistry and has major implications for local and regional air quality. The reaction of isoprene with OH has been investigated by ab initio molecular orbital theory and unimolecular rate theory calculations. We report the energetics of the four possible adducts and the transition states associated with isomerization, which are energetically comparable to the dissociation of the adduct. We have employed CVTST and RRKM/ME theories based on the ab initio calculations to calculate the rates and branching ratios for the OH-isoprene reaction. The calculated fall-off behavior is consistent with recent experimental measurements.