The interlaboratory comparison 'ratio of the contents of the alpha- and beta -Si3N4-phases by X-ray or neutron powder diffraction using the Rietveld method' was launched in early 1999 in connection with a certification project of the BAM, the German national body responsible for quality assurance in analytical chemistry and testing. During the course of this project 46 analytical laboratories in 14 countries have analyzed 285 specimens using the Rietveld method. Among the participants were academic institutions, industrial testing laboratories as well as application laboratories of several instrument manufacturers, applying a large variety of instrumentation, data collection strategies, Rietveld programs and data evaluation strategies.