We have investigated the mechanism of the reaction of H abstraction from n-butanol by the hydroxyl radical (HO center dot) using high level ab initio methods in conjunction with the correlation consistent basis sets up to quadruple-zeta quality (cc-pVQZ). This reaction is of significance in the atmosphere and combustion. The focus of the study has been on the relative importance of the abstractions from the specific n-butanol sites and on the role of reaction intermediates involved. Our results show that abstractions from the C-alpha and C-gamma positions are kinetically most favored and nearly barrierless, with barrier height estimates of 0.10 and 0.47 kcal/mol, respectively, at the CCSD(T)/cc-pVQZ level. We have determined that the indicated barrier height order, C-alpha < C-gamma < C-beta < C-delta < OH, parallels that for the n-butanol bond dissociation energies established recently. The kinetically and thermodynamically most favored C-alpha abstraction occurs via a mechanism including the formation of the n-butanol center dot center dot center dot HO center dot prereaction complex. The weakly bound postreaction complexes between the product radicals and H2O have been identified for all the specific site abstraction reactions, with their calculated CCSD(T) binding energies of up to about 3 kcal/mol after correcting for the basis set superposition error. G3 method has been found to yield consistent results with those obtained from the CCSD(T) calculations for the predicted orders of both the H abstraction barrier heights and their exothermicities.