Abstract: A heat exchanger core is formed from a single foil strip folded into successive U-bends to define alternate fluid passages, with even-numbered passages being for hot fluid and odd-numbered passages being for cold fluid. The folded foil thus defines three walls of each passage with the remaining fourth side open. Top and bottom plates cover and close the open sides of all the hot and cold passages respectively. The ends of certain passages remain open; the ends of other passages are selectively closed by pinching together the ends of adjacent foil walls which define the passages. A pre-formed end plate has apertures between deformable ribs; the apertures are aligned with the ends of open passages, and the ribs overlie and are joined with the pinched ends of the closed passages.