The gas arising from the Claus process that contains sulphur and sulphur dioxide. The gas is treated to remove sulphur vapour, sulphur dioxide, and traces of other sulphur compounds for release to the atmosphere. The recovered sulphur is returned to the process. The treatment involves reducing the sulphur compounds to hydrogen sulphide by passing through a bed of cobalt-molybdenum catalyst. The gases are cooled to remove excess water vapour.