The internal quantum efficiencies of red (∼7000 Å) emitting diodes have been evaluated from measurements of external efficiencies by the method of indices of refraction matching. The internal efficiencies of the diodes are on the average about a factor of five larger than the external efficiencies. The highest internal quantum efficiency was 3.85×10 -2. Free carriers account for the absorption losses of the emitted radiation, but nonradiative recombination processes are responsible for the low values of the internal efficiency. © 1967 The American Institute of Physics.