The effect of radio frequency hydrogen plasma annealing on the interface properties of Si/SiO2 structures is studied by spectroscopic ellipsometry in the spectral range 280-430 nm. By thermal oxidation of (1 1 1)Si at 850 degreesC a 1 nm thick interface with a composition Of Si-0.6(SiO2)(0.4) is formed, which becomes thinner (0.5 nm) and its composition changes to Si-0.8(SiO2)(0.2) after hydrogen plasma annealing. For the as-grown oxide, a shift in the characteristic peaks in the spectrum towards lower energies and a split of the E-1 peak at 3.4 eV are observed and are attributed to the high interfacial stress, the level of which is estimated to be 1.1 X 10(10) dyn/cm(2). These phenomena disappear after annealing of the Si/SiO2 structure. (C) 2003 Elsevier Science B.V. All rights reserved.