Silicon nanocrystals (Si NCs) with average diameters NC>=3, 5 and 8 nm embedded in amorphous SiO2 synthesized by Si ion implantation have been characterized by spectroscopic ellipsometry. The dielectric function of the Si NCs has been extracted from SE data using a Bruggeman effective medium approximation and a Gauss–Lorentz oscillator model, taking into account the size dispersion of the nanoparticles. The dielectric function is found to depend strongly on average NC size. Although electronic critical point transitions of bulk silicon are discernible in NCs down to 3 nm diameter, the E1 resonance weakens with decreasing NC size, nearly disappearing for 3 nm diameter, while the dominant E2 transition blueshifts. In addition, a non-bulk-Si-like resonance intermediate between E1 and E2 is observed.