Measurements of cyclotron resonance absorption have been made in the far infrared spectral region from 25−150 &mgr; on several III–V compounds at room and liquid‐nitrogen temperatures using steady magnetic fields as high as 75 kgauss. Forn‐type InSb, InAs, InP, and GaAs, the data yield information concerning the conduction electron effective mass at the bottom of the band and its variation with magnetic field. Experiments have also been carried out onp‐type InSb and corresponding information has been obtained for light holes. The dependence of the effective masses on both temperature and magnetic field can be satisfactorily interpreted in terms of Kane's theory for the band structure of these materials.Measurements of Faraday rotation and Voigt double refraction have been made in the spectral region between 15 and 25 &mgr; on a number of compound semiconductors at liquid‐nitrogen temperatures. Either experiment gives the effective mass of the free carriers if their concentration is known. If both experiments can be performed, the results can be combined to give both the effective mass and carrier concentration directly without recourse to electrical measurements.