Small angle X-ray scattering by glassy carbon was used to establish the characteristic of submicroscopic porosity. A monochromatized linear beam was used and corrections were made to obtain the scattering curve free of aberration. From analysis of the scattering curve, the voids were deduced to be oblate ellipsoids of revolution. Using the two-density theory, the geometrical parameters and concentration of the voids were determined. Fluctuations of atomic density in the carbon matrix were detected and their magnitude evaluated.