A device for fixing and sealing a replaceable heat-exchanger cassette in an air/air heat exchanger, wherein two spatially separated conducting areas are formed by separating strips and the heat exchanger cassette. In this case a releasable and satisfactory sealing support of the heat exchanger cassette is achieved because in the area of its separating strips the air/air heat exchanger has sealing receivers. The heat exchanger cassette rests against the sealing receivers by sealing faces. The air/air heat exchanger has at least one movable slide which maintains the heat exchanger cassette in the sealing receivers under prestress.