The construction and performance of high-efficiency (95 per cent) counterflow heat exchangers is reported. These exchangers are of the finned-plate type employing interrupted strip fins. Comparison is made between test-core results and performance of large-size units installed in a tonnage gaseous-oxygen plant. Heat-transfer results were in good agreement but measured pressure drops on the large-size units were considerably higher than predicted from test-core results, possibly due to losses at end connections. Successful development of the true counter-flow gas-to-gas exchanger adds flexibility to the design of low-pressure gas cycles as in oxygen plants, permitting the substitution of heat exchangers for regenerators.