A method with associated equipment for feeding two gases into and out of a multi-channel monolithic structure. The two gases will normally be gases with different chemical and/or physical properties. The first gas and the second gas are fed by means of a manifold head into channels for the first and second gases, respectively. The gases are distributed in the monolith in such a way that at least one of the channel walls is a shared or joint wall for both gases. The walls that are joint walls for the two gases will then constitute a contact area between the two gases that is available for mass and/or heat exchange. This means that the gases must be fed into channels that are spread over the entire cross-sectional area of the monolith. The entire contact area or all of the monolith's channel walls are directly used for heat and/or mass transfer between the gases. This means that the channel for one gas will always have the other gas on the other side of its channel walls.