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A fixed-geometry second-throat diffuser system was tested in an open- jet wind tunnel at Mach numbers 3.6 and 7.0. These tests were conducted at Reynolds numbers of 1,000,000 and 200,000, based on expansion nozzle exit diameter. The investigation was conducted to provide diffuser design criteria for the AEDC proposed Hypersonic True Temperature Tunnel (Tripltee). At M infinity = 3.6, empty test section diffuser efficiencies of 96% of normal shock recovery were achieved. Under similar conditions at M infinity = 7.0, efficiencies of 90% were obtained, thus establishing that reasonable diffuser efficiencies can be obtained at both Mach numbers with a single diffuser system. Tunnel operation was possible with conical models of up to 18-% blockage, but diffuser efficiency was reduced by 8 to 12%. Tunnel flow could not be established with models in the test section. Models of 8- to 10-% blockage would not permit establishment of tunnel flow at either M infinity = 3.6 or 7.0. It was necessary to inject the models into the flow after the tunnel was started. Limited diffuser heating rate data was obtained at M infinity = 7.0.