The conduction bands in GaAs and In have been studied very accurately through the cyclotron resonance over a wide range of energies using the photoconductive detection technique. Pronounced band non-parabolicity has been measured in GaAs. Values for the band-edge masses of 0.0660 m₀ and 0.07927 m₀ respectively have been determined. A five-level k.p model of the band structure in the presence of a magnetic field has been used to describe the data where resonant and non-resonant polaron contributions to electron energies are included in the theory. The non-parabolicity and spin-doublet splitting of the cyclotron resonance are well described by the employed formalism in both materials and the theory also gives a reasonable account of the anisotropy of the band in GaAs. The fit to experimental data allowed the authors to determine the matrix element of momentum Q between the higher conduction and valence bands in both materials.