An experimental investigation of separated boundary-layer flow was conducted on a two-dimensional flat plate having a variable-span trailing edge flap in the 12-in. Supersonic Tunnel (D) of the von Karman Gas Dynamics Facility. The tests were made at Mach numers 3 and 5 at zero angle of attack and over a Reynolds number range (based on plate length) from 0.26 to 16.7 x 10 to the 6th power. Model surface pressure distributions, schlieren photographs, and velocity distributions are presented. The effects of unit Reynolds number, plate length, flap span, and flap deflection angle on the separated region are investigated, and comparisons are made with existing theory.