We have measured the effect of white light illumination and annealing to 180°C on the field effect conductance of glow discharge α-Si : H. Annealing produces a change in the off conductance by up to a factor of 30, and moves the threshold field by 2 x 104 Vcm-1. These changes are reproducibly reversed upon illuminating with white light. A major part of the effect of illumination is most likely due to a movement of the Fermi level in the bulk of the material. Annealing to 180°C reverses this effect. Furthermore excess electrons can be trapped at the amorphous silicon-silicon nitride interface by a field assisted trapping mechanism and these are also de-trapped upon annealing to 180°C