An experimental study of film cooling effects produced by injection of a helium secondary flow into a low speed air mainstream is presented. The helium was injected through a porous section into a turbulent boundary layer flowing over a flat plate. For comparison, measurements were also made using air as the secondary fluid. The adiabatic wall temperature, presented in a dimensionless group known as the film cooling effectiveness, was measured for both helium and air secondary flows. The experimental data are compared with predictions. It is found that helium injection produces higher film cooling effectiveness than anticipated. The measured helium concentration along the wall is in relatively good agreement with predictions. Temperature and helium concentration profiles are also presented. The concentration and temperature profiles are very similar and in good agreement with profiles measured with air injection.