The properties of n-type (111) Si surfaces in aqueous 0.1 M K4Fe(CN)(6) + 0.5 M KCl at pH 9 were studied both in the dark and under illumination. In this solution, oxide passivation of the surface gives rise to a variety of electrically active interface states. Both in the dark and under illumination, interface states interacting with the conduction band are present at an energy of about 0.36 eV below the conduction bandedge, giving rise to a characteristic impedance at potentials close to the flatband potential. Furthermore, interface states which act as recombination centers are observed when the surface is illuminated. The density of recombination centers was found to be a function of the light intensity, ranging from 1 x 10(12) to 3 x 10(12) cm(-2), indicating that these states are related to oxidation intermediates. The rate constant for recombination was determined to be about 3 x 10(-9) cm(3) s(-1).