The S-34 and C-13 isotopic fractionations in the OCS + OH reaction are investigated using RRKM-theory. The reaction proceeds via different channels, at low pressures a channel leading to CO + SOH dominates. While with increasing pressure an adduct forming channel becomes competitive. The resulting overall rate constant is not strongly dependent on pressure and agrees well with experiments. The competition between channels makes the fractionation constants highly pressure dependent, however the sulfur-34 fractionation constant varies only from about -5 parts per thousand to 0 parts per thousand in the troposphere and lower stratosphere. The carbon-13 isotopic fractionation is stronger; between -40 parts per thousand and -70 parts per thousand in the troposphere and lower stratosphere. (C) 2012 Elsevier B.V. All rights reserved.