Thermoelectric power and conductivity measurements have been made on a series of amorphous silicon films illuminated with 633nm light. At high temperatures where the photoconductivity was small compared with the dark conductivity the thermoelectric power under illumination was what is normally expected in the dark. As the temperature was lowered the thermoelectric power under illumination reversed sign in all the films investigated. In a preliminary analysis it is concluded that it is difficult to interpret the data quantitatively and that further work is required to establish the true nature of this effect.