Abstract: Reactor for removing oxidizable pollutants from a waste gas stream which comprises (1) means for introducing the waste gas stream through an inlet into a chamber containing a substantially horizontal catalyst bed container means below the inlet, which container means is for containing a granular catalyst bed for oxidizing the pollutants and which container means comprises (a) support means for the catalyst bed which support means is permeable to the waste gas stream and covers substantially the entire horizontal cross section area of the chamber below the catalyst bed and has an outer edge adjacent to the chamber wall, (b) packing means to fit around the outer edge of the support means to prevent loss of catalyst between the edge of the support means and the wall of the chamber, (c) ledge means which provides a substantially horizontal support around the inside periphery of the chamber wall for leveling the catalyst bed and which ledge means also exerts substantial downward and outward pressure on the packing