The effects of illumination with white light and annealing to 180-200 degrees C on the field-effect conductance of a-Si:H FETS have been investigated. The effects produced by illumination are completely reversed by annealing. The authors find that illumination produces two types of change in the field-effect conductance; first a maximum decrease in the off-conductance of about 300%, with a positive shift of the threshold voltage' and secondly a horizontal displacement of the conductance characteristic in the pre-threshold region with no change in the off-conductance. In both cases there is a movement of the bulk Fermi level with respect to the conduction band mobility edge. These two effects may be produced in a single silicon film sample.