Abstract: The present invention is a flat stack permeator useful under dialysis, ultrafiltration, reverse osmosis, perstraction, pervaporation, etc. conditions. The permeator comprises a multitude of membrane layer alternately separated by feed-retentate spacers and permeate spacers. The layers are secured along their edges so as to define separate feed-retentate zones and permeate zone. The edge gluing is performed so that in any given layer the two parallel edges are secured, while on the layers immediately above and below the edge pairs which are secured are 90.degree. out of register with the previously mentioned pair of secured edges. In that way alternate feed retentate and permeate zones are defined which are perpendicular in flow one to the other.