The reaction of O(P-3) with isobutene ((CH3)(2)C=CH2) is investigated using the unrestricted second-order Mlller-Plesset perturbation (UMP2) and complete basis set CBS-4M level methods. The minimum energy crossing point ( MECP) between the singlet and triplet potential energy surfaces is located using the Newton-Lagrange method, and it is shown that the MECP plays a key role. The calculational results indicate that the site selectivity of the addition of O(P-3) to either carbon atom of the double bond of isobutene is weak, and the major product channels are CH2C(O)CH3 + CH3, cis-/trans-CH3CHCHO + CH3, (CH3)(2)CCO + H-2, and CH3C(CH2)(2) + OH, among which (CH3)(2)CCO + H-2 is predicted to be the energetically most favorable one. The complex multichannel reaction mechanisms are revealed, and the observations in several recent experiments could be rationalized on the basis of the present calculations. The formation mechanisms of butenols are also discussed.