The properties of electropolishing layers formed on illuminated n-Si(111) in dilute NH4F are analysed using a combined (photo) electrochemistry/UHV surface analysis system. Evaluation of thickness d and composition dependencies on potential U and pH shows increasing d at lower pH (d = 25 Angstrom at 3 V, pH 3) and for U > 3 V at pH 4.9 (d = 7 Angstrom, U = 5 V). Fluorides, oxyfluorides, hydroxides and oxides of Si are found in the films. The surprising observation that thicker films transmit higher currents is discussed. UP spectra (He II) reveal a valence band offset between Si and the oxidic layer of 5.2 eV.