The sulfur kinetic isotope effect (KIE) in the reaction of carbonyl sulfide (OCS) with O(P-3) was studied in relative rate experiments at 298 +/- 2 K and 955 +/- 10 mbar. The reaction was carried out in a photochemical reactor using long path FTIR detection, and data were analyzed using a nonlinear least-squares spectral fitting procedure with line parameters from the HITRAN database. The ratio of the rate of the reaction of (OCS)-S-34 relative to (OCS)-S-32 was found to be 0.9783 +/- 0.0062 ((34)epsilon = (-21.7 +/- 6.2)parts per thousand). The KIE was also calculated using quantum chemistry and classical transition state theory; at 300 K, the isotopic fractionation was found to be (34)epsilon = -14.8 parts per thousand. The OCS sink reaction with O(P-3) cannot explain the large fractionation in S-34, over +73 parts per thousand, indicated by remote sensing data. In addition, (34)epsilon in OCS photolysis and OH oxidation are not larger than 10 parts per thousand, indicating that, on the basis of isotopic analysis, OCS is an acceptable source of background stratospheric sulfate aerosol.