Mineral modes have been determined for specimens of five sedimentary rocks (arenite, graywacke, slate, mudstone, and shale) from Cu Kalpha X-ray powder diffraction data using the Rietveld method. Up to eight individual mineral components were measured in each sample, with a detection limit of about 0.5 wt %, depending on the mineral assemblage. The X-ray results, when expressed as reduced oxides (wt %), are in reasonable agreement with XRF oxide compositions. The power of the Rietveld method in this application to mineral mode determination is its ability to (1) cope with complex diffraction patterns containing substantial peak overlap, (2) provide modes that do not generally require independent, lengthy calibration, (3) determine the chemical composition of the major phases during the analysis (thereby providing a degree of self-calibration), and (4) deal with very fine-grained samples. Disadvantages include (1) the need to know the identity of all the components beforehand, and (2) the need for experience in X-ray diffraction analysis by the Rietveld method.