Abstract Infrared refectivity and magnetoreflectivity measurement were performed at 30 °K on‐type PbSe samples with electron concentrations rangingfrom 4.5 × 10 17 to 4.3 × 10 18 cm −3 . From the best fit of experimental and theoretical curves the electron effective mass and relaxation time were determined as functions of electron concentration. The dependence of the effective mass on electron concentration is in good agreement with Kane's model of the conduction band.