Photocurrent oscillations at n-type Si(111) were investigated in ammonium fluoride solutions by in situ ellipsometry and ex situ transmission electron microscopy (TEM) experiments. In situ ellipsometric data show that the current oscillations are concomitant with fluctuation of the optical parameters Delta and Psi. TEM analysis revealed the presence of pores in the silicon oxides during the oscillations. Effective medium theory following Bruggemann’s formalism was used to evaluate the change in refractive index and thickness of the anodic oxide film. The observed fluctuation in Delta and Psi can be explained by changes in refractive index of the oxide due to the opening and closing of pores.