The mechanism of the reaction between O(D-1) and NF3, experimentally studied by spectroscopic techniques [V.I. Sorokin, N.P. Gritsan, A.I. Chichinin, J. Chem. Phys. 108 (1998) 8995], has been investigated at the Coupled Cluster level of theory in conjunction with double-zeta and triple-zeta quality basis sets. The process commences by the ex-oergir (105.4 kcal mol(-1)) formation, on the singlet surface, of the O-NF3 intermediate, whose eventual dissociation into NF2 and OF passes through the isomerization to F2N-OF. The energy barrier of this process, 58.8 kcal mol(-1), is significantly lower than the intersystem crossing from the singlet O-NF3 to the triplet O(P-3) and NF3 dissociation products. This is consistent with the experimental observation that, in the reaction between O(D-1) and NF3, the unreactive quenching to O(P-3) represents only a minor reaction channel. (C) 2002 Elsevier Science B.V. All rights reserved.