In this paper the particle size distribution of SiO(2) spherical nanoparticles is determined by small angle X-ray scattering (SAXS). The experimental scattering intensity data were compared with theoretical intensities based on different numeric procedures and different assumptions for the distribution of the particles. Hereby two fixed shapes of distribution, Gauss and log normal, were used and compared to a free particle distribution which is determined with the maximum entropy method (MEM.). The results were further compared with the particle size distributions obtained from the transmission electron microscopy (TEM) and analytical ultracentrifuge (AUC). The comparison demonstrated SAXS to be a generally reliable method for determining size distributions of spherical particles in the nanometer range. Assuming a fixed distribution in the SAXS method, reliable data can be obtained even in the case of bad statistical data or higher concentrations. The MEM procedure allows shape determination but is restricted to scattering data of dilute samples and high statistical quality if one wishes to avoid artefacts in the resulting size distribution.