Measurements of the effective mobility, field effect mobility, Hall mobility, and carrier density of Si as a function of field perpendicular to the surface are reported. At all temperatures from 4.2 to 300°K, at least one maximum in the mobility was observed. The temperature dependence is reported for different fields. At room temperature, a single maximum in the mobility was observed close to the threshold for inversion. As the temperature was lowered, this peak increased. At temperatures near 80°K, it then decreased. Another maximum appeared at about 100°K at higher fields; it increased as the temperature was lowered. An anomalous shift in the conductance threshold between 77.3 and 4.2°K is reported and is correlated with the charge in the oxide. Effects of substrate bias are reported. Some comments are made on possible scattering mechanisms. The effect of interface states was measured and their density near the conduction band is reported. © 1968 The American Physical Society.