The reaction mechanisms of the (1)Sigma(+) ground state of ScS+ with oxygen-transfer reagent: ScS+ + COS --> ScO+ + CS2 in the gas phase has been studied by using ab initio methods. It is found that the reaction proceeds via two four-center transition states (TS1 and TS2) with a cyclic complex (b) located between them on the reaction potential surface. The first reaction step proceeding via TS1 is rate limiting and the activation barriers are 31.5 and 6.4 kcal/mol at MP4 (SDTQ)/6-31+G*//MP2/6-31+G* level plus ZPE, relative to the intermediate complex (a) and the reactants, respectively. (C) 2002 Elsevier Science B.V. All rights reserved.