X-ray powder diffraction provides a direct analysis of phase abundance of portland cement clinkers and cements. The ASTM C01 (Cement) Committee formed a task group to investigate quantitative X-ray powder diffraction analysis that resulted in the ASTM C 1365 standard test for X-ray powder diffraction of portland cement clinker and portland cement. Estimates of test precision and bias are required in ASTM test methods with round robin testing used to generate these data. Seven laboratories participated in a clinker round robin using the NIST Standard Reference Material (SRM) clinkers and Rietveld analysis. The results were used to estimate inter- and intra-laboratory precision and bias. The application of the Rietveld method for analysis of complex powder diffraction patterns of clinker and cements provides an improved means to measure individual phase pattern intensities by accommodating the intrinsic variability of parameters affected by chemical substitution. Results indicate an improvement over round robins conducted prior to use of Rietveld method in cements analysis. The standard test method is being modified to include this approach.