The fractionation of sulfur isotopes in the gas-phase reaction of OCS with OH was calculated by ab initio methods. The first reaction step occurs through a carbon bonded OH-OCS transition state. The activation barrier is 18.4 kJ mol(-1) (CCSD(T)/aug-cc-pVDZ level plus zero-point energy) relative to the reactants. Fractionation constants are -1.4, -2.6 and -6.3 parts per thousand for S-33, S-34 and S-36 respectively. These results bring new insight regarding the origin of stratospheric sulfate aerosols, including the identification of characteristic isotopic fractionations in the lower and middle stratosphere. (C) 2007 Elsevier B.V. All rights reserved.