Film cooling downstream of secondary gas injection through discrete holes has been studied experimentally. The influences of hole geometry, secondary fluid density, and mainstream boundary layer thickness are described. Significant improvements in the film cooling effectiveness are observed by having the coolant passages widened before the exit of the secondary fluid. The use of a relatively dense secondary fluid, as might be encountered in many applications, requires a significantly higher blowing rate to cause jet separation from the surface than when the densities of the freestream and secondary stream are the same. This results in considerably better film cooling over an important range of density ratios.