A supercritical airfoil with a trailing edge flap has been the subject of aerodynamic investigation in the National Aeronautical Establishment's High Reynolds Number 2-D Test Facility. The effects of flap deflection on buffet intensities and delay of buffet onset at transonic speeds were studied. Buffet boundaries for various flap angles were determined from the divergence of the fluctuating balance normal force measurements (CN). The onset of buffet was obtained from plots of C/N versus C/L at values of C/L where the slope was 0.1. This value for the slope was arbitrarily chosen, but was found to give consistent results which agreed well with values computed from the criterion using the trailing edge pressure divergence for those cases where buffet onset is primarily due to trailing edge separation. For given flow conditions the deflection of the trailing edge flap altered the circulation and hence the position of the shock wave and its strength. The variations of these two quantities with flap angles were determined from pressure measurements carried out on the airfoil surface. The test was performed quite deep inside the buffet régime and shock wave oscillations were investigated from spectral analyses of the balance outputs. Fluctuations in the shock motion of approximately 70 Hz were detected. The drag of the airfoil was computed from the wake stagnation pressure deficit and the drag penalties for large flap angles were quite significant. Analysis of the wake profiles with flap settings was carried out to study the changes in wake characteristics. This investigation shows that trailing edge flap is a useful device for passive buffet aIleviation of supercritical airfoils. For Mach numbers less than the design values, the lift coefficients at buffet onset are very close to and in some cases correspond to C/L max. This is quite unlike conventional airfoils when regions of mild, moderate and heavy buffet can be detected before C/Lmax is reached.