A catalytic reactor comprises a plurality of sheets defining flow channels between then. Within each flow channel is a foil of corrugated material whose surfaces are coated with catalytic material. The flow channels extend in transverse dire options, but the foils are shaped to cause the gas in those channels to flow at least partly in counter current to the gas flowing in the other channels. The reactor incorporates header chambers to supply gas mixtures to the flow channels, each header being in the form of a cap attached to the outside of the back and covering a face of the stack. Hence different gas mixtures are supplied to the different channels which may be at different pressures, and the corresponding chemical reactions are also different, and heat is transferred through the sheets separating the adjacent channels. When the catalyst in one set of flow channels becomes spent, it can be replaced by removing a header.