Theoretical studies of the unimolecular and water-assisted decomposition reactions of the a-hydroxy hydroperoxides compounds produced during the isoprene ozonolysis have been investigated in this paper. Geometrical parameters of all the stationary points as well as energies and rate constants have been computed by means of several ab initio and DFT methods (B3LYP, CCSD(T), and G2M-RCC5 levels). Our calculations indicate that only the water-assisted decomposition of the a-hydroxy hydroperoxides could be active in the atmosphere. The main reaction products are predicted to be H2O2 Plus methyl vinyl ketone (MVK) or methacroleine (MAC), depending on the particular (x-hydroxy hydroperoxide considered. In both cases the reaction is endothermic by about 19 kcal mol(-1).