Magnetoreflection experiments involving both intraband and interband transitions can provide valuable information about the electronic band structure of semiconductors. In the intraband experiments, performed near the plasma reflection edge, the application of a magnetic field splits the edge and results in the formation of two minima separated by the cyclotron frequency. It is thus possible to determine the cyclotron frequency directly, at room temperature, and for high carrier concentrations. When scattering losses are taken into account in the theory, it becomes possible to determine the carrier concentration, scattering time, and effective mass from the optical measurements alone. The theory of the effect is discussed for applied magnetic field transverse and parallel to the direction of propagation, and experimental results are presented for InSb, InAs, and HgSe. A consistent fit to Kane's theory for the variation of mass with concentration in InSb is obtained when previously published data have been corrected for reststrahl dispersion.Interband magnetoreflection experiments can be useful in cases where high absorption coefficients or difficulty of preparing thin samples make transmission experiments unfavorable. This type of experiment yields information on energy band gaps, effective masses, andgfactors. Experimental data for the direct transition in InSb are presented.