The competing pathways of H-abstraction by oxygen molecules and 1,5 H-shift of the delta-alkoxy radicals with the Z-configuration arising from OH-initiated reactions of isoprene have been investigated using density functional theory (DFT) and ab initio molecular orbital calculations. The activation and reaction energies of the alkoxy radical reactions were obtained with B3LYP, CCSD(T), and MPW1K and various basis sets. Kinetic calculations employing variational RRKM/ME formalism and separate statistical ensemble (SSE) theory show that a significant fraction of the chemically excited alkoxy radicals undergo prompt 1,5 H-shift. The results also reveal that 1,5 H-shift of thermalized delta-alkoxy radicals dominates over H-abstraction by O-2. (C) 2003 Elsevier Science B.V. All rights reserved.