An exhaust gas treatment device for internal combustion engines and the like includes inlet and outlet end caps, two catalyst substrates, and a two-piece housing. A first, cylindrically-shaped housing member has a hollow interior in which one of the substrates is retained, a first end sealingly connected with the inlet end cap, and an opposite second end with a radially reduced section. A second cylindrically-shaped housing member has a hollow interior in which the other one of the substrates is retained, a first end sealingly connected with the outlet end cap, and an opposite second end with a radially enlarged section sized to receive therein the second end of the first housing member, whereby the reduced section of the first housing member and the enlarge section of the second housing member are spaced radially apart a predetermined distance to define an annularly-shaped space or gap which thermally insulates the associated portion of the exhaust gas treatment device.