The potential dependence in the surface second harmonic response from hydrogen terminated n-Si(111) and oxidized n-Si(111) surfaces has been examined in aqueous NH4F and H2SO4 solutions. The relative phase of the nonlinear response as measured by rotational anisotropy experiments is found to be highly sensitive to the presence of the oxide and the field applied across the Si(111)/oxide/electrolyte interface. These observations are attributed to field effects within the space-charge region of the semiconductor which vary with the presence and thickness of the insulating oxide layer on the Si(111) surface.