Anomalous small-angle X-ray scattering(ASAXS) was used to determine the Ni particle size distribution of an Ni/SiO2 catalyst. In the analysis of the data no assumptions were made regarding the form of the size distribution. Parameters calculated from the distribution were in good agreement with the mean crystallite size estimated from the line broadening of X-ray diffraction peaks. Our results indicate that ASAXS will become a very useful tool for the determination of particle size distributions in catalysis research.