The propagation of light emitted from silicon nanocrystals forming planar waveguides buried in SiO2 is studied both experimentally and theoretically. Experiments reveal that photoluminescence spectra detected from the sample facet mainly contains narrow (10–20 nm full-with-at-half-maximum) polarization-resolved transverse electric and transverse magnetic modes instead of the usual broad nanocrystal emission band peaked at 700–800 nm. A theoretical model developed in the framework of wave optics identifies these modes as substrate modes propagating along the waveguide boundary (not the usual modes guided inside the nanocrystal plane due to its graded index profile). This peculiar observation is the consequence of the high losses in the nanocrystalline waveguide and may occur in other dissipative waveguide structures under conditions that are discussed.