A supercritical airfoil with a trailing-edge flap was tested at the High Reynolds Number Two-Dimensional Test Facility of the National Aeronautical Establishment. Buffet boundaries at various flap angles were determined from the divergence of the indicated normal-force fluctuations. The test was performed quite deep into the buffet regime of the airfoil, and spectral analyses of the unsteady normal force showed shock-wave oscillations of approximately 50-80 Hz for Mach numbers between 0.612 and 0.792. The drag of the airfoil was measured from wake probes, and the drag penalties for the large flap angles were quite significant. This study illustrates the use of trailing-edge flaps for buffet alleviation at transonic conditions.