The published fixed-wavelength nulling ellipsometric data on chemically cleaned silicon (Si) surfaces are re-interpreted in terms of error analyses using a three-phase model (air - layer - substrate). The effective thickness of the surface layer formed by the chemical treatment as well as the angle of incidence at which the surface ellipsometric data have been measured are evaluated from this model in which the surface layer is considered an equivalent dielectric film of known refractive index. The error analyses which accompany the computation procedure show that such ultra-thin surface layer thicknesses can be evaluated with sufficient accuracy in spite of the inaccurate knowledge of the layer index, from nulling ellipsometers producing angular errors of a few hundredths of a degree. The correlation between the angle of incidence and the substrate's real index value found allows the latter to be corrected, provided that the angle of incidence is measured directly with sufficient accuracy. The layer thickness values are deduced from the null-ellipsometric data measured for Si(100) surfaces subjected to different chemical treatments and are compared with those found by spectroscopic ellipsometry. Mainly layer thicknesses of about 6.5 and 12 Å are detected respectively on HF-treated and SC1 ()-oxidized Si wafers. It is thus shown that preliminary results on the effect of chemical treatment of substrates could be obtained more rapidly from fixed-wavelength nulling ellipsometric measurements than they could by spectroscopic ellipsometric methods.