The infrared absorption of CdO-films with electron concentrations from 1015 to 1020 cm-3 was measured at 300 and 85°K up to 50μm. With increasing carrier density, the 'classical' spectral dependence of the electron and lattice absorption is considerably modified. At the short-wave side of the lattice band two sidebands appear. An additional absorption above the LO-lattice frequency can be explained according to the theory of Gurevich by the coupling between electrons and LO-phonons. © 1969.