A diffuser of variable geometric shape was tested for pressure recovery at M = 1.9, 2.5, 2.8, and 4.9 in the continuous 18- x 18-cm Aerophysics tunnel no. 3. The diffuser consisted of a converging-diverging, 2-dimensional, variable-area duct. The opening of the diffuser throat and the location of the throat along the center line of the duct were varied systematically. The effect of closed or half-open test section on diffuser efficiency and that of a model in the air stream were investigated. Operating and starting pressure ratios of the wind tunnel were determined, and performance data of the diffuser are given for both cases. Pressure recoveries of the operating tunnel were obtained for optimum diffuser configuration with diffuser end-pressure ranging up to 1.8 times pitot pressure in the test section at M = 4.9. Also lower pressure ratios to start the tunnel than previously known were found. Using this diffuser, a continuous tunnel can be operated with less power, or an intermittent tunnel can run for a longer time, and they can be started with a smaller (closer to one) over-all pressure ratio. The pressure recoveries obtained in a half-open test section and with models in the stream are somewhat lower, but the performance of the diffuser is not seriously offset by those modifications.