By studying the photovoltage developed across the amorphous silicon‐electrolyte interface on illumination with monochromatic light, the mobility gap in amorphous silicon is found to be about 1.7 eV. The effect of preparation conditions is very significant, with unannealed films deposited by electron beam evaporation showing considerable structure in the spectral response. After low‐temperature annealing, typically at 120°C for 20 h, the spectral response shows a sharp edge and is similar to that of single‐crystal silicon but is displaced to higher energies by about 0.6 eV. Sputtered films, even when unannealed, show almost identical spectral response as fully annealed films, which is attributed to the process of film growth in the sputtering process.