Abstract: An apparatus for effecting high temperature catalytic conversion treatment of combustion exhaust gasses flowing in an exhaust path of an industrial installation which comprises a housing assembly including a fixed housing unit forming a fixed part of a gas exhaust path defining an opening, the housing assembly includes a cover member mounted on the fixed housing unit for movement from an operative position closing the opening to an open position providing access to an interior space within the fixed housing unit through the opening. A mounting assembly is provided for mounting one or more catalytic carrier assemblies stably both axially and peripherally within the housing assembly so that exhaust gasses flow only through passages in the catalytic carrier assemblies in operation.