Thin film transistors were prepared using evaporated PbS films. It is shown that films containing oxidised barriers to the conductivity have very little sensitivity to electric fields. However, nearly intrinsic films show a much larger field effect, and can be switched from n-type to p-type conduction by the field. Effective electron mobilities as high as 260 cm2/V-sec were obtained. Interesting effects due to non-uniform fields applied by the gate are illustrated and discussed. While the data can be qualitatively explained in terms of a simple model, the strong frequency dependence is not yet understood. The sensitivity to water vapor indicates, however, that surface states may play a strong role. Mobility variation and capacitive relaxation may also occur. © 1965.