We report on the results of small angle x-ray scattering and high resolution electron micrograph studies which reveal the structure of hydrothermally prepared porous silicon that emits blue light under photoexcitation. Our results constitute direct evidence that blue photoluminescence porous silicon includes high density silicon quantum dots. We show that these quantum dots are isolatedly dispersed in each amorphousSiO2linear column standing freely on the silicon substrate. The size of individual quantum dots is about 2 nm. ©1997 American Institute of Physics.