The quantitative reliability of theoretical expressions for Hall and thermoelectric effects has been tested under thermal equilibrium conditions in high‐conductivity, nondegenerate, and degenerate CdS single crystals, and under nonequilibrium steady‐state and transient conditions in low‐conductivity photosensitive CdS crystals. The thermal equilibrium measurements were made as a function of temperature, hence of scattering and degeneracy, and the nonequilibrium measurements were made as a function of excitation intensity and temperature. Good agreement is found in every case between the values of the electron density and mobility calculated from the Hall effect and the values of these quantities calculated from the thermoelectric effect. The results confirm both the reliability of the present state of the theory for materials like CdS, and the extension of this theory far from equilibrium conditions provided that the equilibrium Fermi level is replaced by the steady‐state or quasi‐Fermi level.