Abstract
In a series of gas mass-spectrometric measurements performed near the highest attainable accuracy, samples from two highly homogeneous batches of silicon crystals and silica powder were compared directly with a synthetic mixture of the three stable isotopes of silicon. Thereby, this work not only established the "absolute" atomic weight of these batches, but also makes portions of these batches available as an Isotopie Reference Material for accurate isotopic abundance measurements in geochemical and other isotope-abundance studies of silicon.