A chemical reactor has two chambers (1,3) separated by a partition wall (5), a porous heat radiating body (2) being disposed in one of the chambers (1) and a porous heat-receiving body (4) being disposed in the other chamber (3). The heat-receiving body (4) is divided into three layers (4a,4b,4c) in the direction of flow of the raw gas mixture through it and the porosity of the layers increases in the direction of flow of the gas. The resultant differential ability to absorb the radiant heat means that there is a substantially linear temperature differential through the heat-receiving body whereby the catalyst material dispersed in it can operate effectively substantially throughout its entire volume.