Some results are presented from the first tests in a program to determine the most efficient diffuser configuration for use in a supersonic wind tunnel. Sufficient data have been obtained at M 2.48 and M 2.83 to make possible the design of a simple yet very efficient diffuser to be used at these Mach numbers in a supersonic wind tunnel with a square cross-section. It has been found that a very efficient tunnel system can be obtained using a simple converging-diverging, variable-throat diffuser if the diffuser throat is located at a point approximately 6.7 times the nozzle exit opening from the nozzle exit. This diffuser should start at a point approximately 1.4 times the nozzle exit opening.