Sulfur is an important element in some food chains and the release of radioactive sulfur to the environment must be closely controlled if the chemical form is such that it is available or potentially available for entering food chains. The presence of 35S in the coolant gas of the Windscale Advanced Gas-Cooled Reactor (WAGR) warranted a study to assess the quantity and chemical form of the radioactive sulfur, in order to estimate the magnitude of the potential environmental hazard that might arise from the release of coolant gas from civil advanced gas-cooled reactors. A combination of gas chromatographic and radiochemical analyses revealed carbonyl sulfide to be probably the only 35S compound present in the coolant gas of the WAGR. The concentration of carbonyl sulfide was found to lie in the range 40 to 100 X 10-9 parts by volume and the 35S specific activity was ∼740 X109 Bq· kg-1 (20 mCi/g). The 35S appears to be derived from the sulfur and chlorine impurities in the graphite although other sources cannot be ruled out. Carbonyl sulfide labeled with 35S can be prepared in the laboratory from KCN 35S.