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Silicon-on-insulator (SOI) optical waveguides insure high electromagnetic field confinement but suffer both from sidewall roughness responsible of scattering effects and from leakage toward the silicon substrate. These two mechanisms are the main sources of loss in such optical waveguides. Considering the case of single-mode ultrasmall square SOI waveguides, propagation loss is calculated at telecommunication wavelengths taking into account these two loss contributions. Leakage toward the substrate and scattering effects strongly depend on the waveguide size as well as on the operating wavelength.