This Technical Specification specifies the method for determining particle size distribution of ultra-fine powders by the small angle X-ray scattering technique. It is applicable to particle sizes ranging from 1 nm to 300 nm. In the data analysis, it is assumed that particles are isotropic and spherically shaped. The method described in this Technical Specification is also applicable to particle suspensions. This Technical Specification does not apply to: a) powders containing particles whose morphology is far from spherical, except by special agreement; b) powders consisting of porous particles; c) mixtures of powders.