The rate constant of the reaction between OH and OCS in helium over the temperature range 255–483 K has been determined using the discharge flow‐resonance fluorescence technique. The OCS has been carefully purified to avoid interference from H2S and CO impurities. An FTIR with a multireflection cell was used to determine the impurity concentrations and the purified sample was found to contain less than 0.005% of H2S. At 300 K, the rate constant was determined to be (2.0 ±0.4 0.8) × 10−15 cm3 molecule−1 s−1. Although the rate constants showed slight positive deviation at lower temperatures, thev can be satisfactorily fitted by the Arrhenius equation, k = 1.13 × 10−13 exp(−1200/T) cm3 molecule−1 s−1. No pressure dependence was observed at all temperatures, nor was O2 enhancement observed under our experimental conditions.