Apparatus for treating a gas stream comprises a plurality of cylindrical proton conducting membranes. The gas stream is conveyed through the bore of each cylindrical membrane, and a hydrogen-containing gas is conveyed about the external surface of each cylindrical membrane. A catalyst provided on the inner side of each membrane catalyses a reaction between a halogen-containing component of the gas stream, such as CF4, adsorbed on the surface of the catalyst and protons conducted through the membrane.