Abstract: A catalytic chemical process is carried out non-adiabatically using as a catalyst a body with two sets of channels throughout the body. One set of channels leads the process fluid towards one of two parallel walls of the catalyst chamber, the other set towards the outer wall, In slits between walls and catalyst heat is exchanged between process fluid and at least one of these walls upon reflecting the process fluid leaving channels of one set and entering channels of the other. The body may be made of alternatingly corrugated and plane sheets using in turn two different orientations for the corrugated sheets. It is preferred that the sheets are arranged orthogonal to the heat transmitting wall(s) and parallel to the overall direction of flow. The method is preferred for endothermic processes, especially steam reforming of hydrocarbon(s).