The size distribution, band gap energy, and photoluminescence of silicon nanocrystals embedded in SiO2 have been measured by direct and independent methods. The size distribution is measured by coupling high-resolution and conventional electron microscopy in special imaging conditions. The band gap is calculated from photoluminescence excitation measurements and agrees with theoretical predictions. Their correlation allows us to report the experimental Stokes shift between absorption and emission, which is 0.26±0.03 eV, independent of average size. This is almost exactly twice the energy of the Si–O vibration (0.134 eV). These results suggest that the dominant emission is a fundamental transition spatially located at the Si–SiO2 interface with the assistance of a local Si–O vibration. © 2000 American Institute of Physics.