The structure and energetics for the reaction of OH +/-isoprene --> adduct have been examined using ab initio molecular orbital methods. The structure of each HO-isoprene adduct was optimized using Becke's three-parameter hybrid method employing the LYP correction functional (B3LYP) with the 6-31G** basis set, and using Moller-Plesset correlation energy correction truncated at second-order (MP2) with both the 6-31G** and the 6-311G** basis sets. Single-point energy calculations using fourth-order Moller-Plesset perturbation theory including single, double, triple, and quadruple excitations, as well as spin projection (PMP4(SDTQ)) with the 6-311G** basis set, were carried at these optimized geometries. The single-point energy was further corrected with zero-point energy (ZPE) to assess the stability of the OH-isoprene adducts. At the PMP4-(SDTQ)/6-311**//mp2/6-311** + Delta ZPE level of theory addition of OH to the 1 and 4 carbons of isoprene produces adducts which are 37.9 and 35.4 kcal mol(-1) (respectively) more stable than the OH and isoprene reactants, while addition of OH to the 2 and 3 carbons results in adducts which are 25.6 and 24.2 kcal mol(-1) more stable than the reactants. Experimental detection of the products from the OH + isoprene reaction using a discharge-flow system coupled with a mass spectrometer shows evidence for the production of all four possible adducts. These results suggest that each adduct is formed with nonnegligible yields, allowing each to participate in subsequent steps in the OH-initiated oxidation of isoprene.