An investigation of supersonic diffusers was conducted for conditions wherein a boundary layer was present at the diffuser entrance. The pressure rise obtained from a shock in a constant-area tube was measured, and the measurements compared with values computed from a simple one-dimensional analysis. For the range of Mach numbers covered (1.8 to 4.2), separation of the stream from the tube wall was always induced by the shock. The length of the separated region was from 8 to 12 tube diameters. The results of the tests on shocks in a constant-area tube were utilized in the design of several types of diffusers for which test data are reported at Mach numbers ranging from 2 to 3. The most efficient diffuser tested consisted of a contraction to a minimum starting area followed by a constant-area tube approximately 10 diam long. The tube was in turn followed by a subsonic diffuser. Only diffusers of constant geometry were considered.