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Surface photovoltage (SPV) in hydrogenated amorphous silicon (a‐Si:H) has been measured as a function of temperature and wavelength, intensity and chopping frequency of light. Effects of surface treatments on SPV, including etching and exposure to moisture, are also studied. It is concluded that SPV cannot be directly related to the band bending at the surface of a‐Si:H. By solving Poisson’s equation in dark and light, it is shown that in a material such as a‐Si:H, the observation of a finite SPV necessarily implies a transfer of charge between the surface states and the space‐charge region. This explains our experimental observations since it means that SPV measurement may not be relied upon to give the surface potential of a‐Si:H. Notwithstanding this handicap, it is shown that by measuring SPV and conductance on an a‐Si:H sample subjected to cycles of moisture and light soaking, we can conclude that light soaking affects the surface as well as the bulk of a‐Si:H.