X-ray powder diffraction analysis is one of the most common methods for determining the structure of crystalline materials, an essential measurement for the development and manufacture of products containing such materials. There are an estimated 20,000 X-ray diffractometers worldwide that are used daily in virtually every materials research and development laboratory. Calibration of these instruments is critical to attain the measurement accuracy and precision needed for a myriad of applications. Structure Determination Methods Group researchers produce a suite of X-ray diffraction SRMs that are broadly disseminated via sales to instrument vendors and end users with existing instruments. Recently, the accuracy and precision of two of these SRMs were significantly improved. Both SRMs are comprised of alumina (Al2O3), in the form of a powder (SRM 676a), and as a sintered plate (SRM 1976a). The certification of SRM 676a for crystalline phase purity will now enable accurate measurement of the thickness of disordered surface layers, which is of particular importance for nanoscale materials with surface-dependent properties. Data for this SRM was collected via neutron time-of-flight scattering and via X-ray diffraction at energies of 25 and 67 KeV; analyses were carried out using the quantitative Rietveld method. SRM 1976 has become the largest selling SRM for X-ray powder diffraction as it is offered in an “indestructible” form and permits calibration of laboratory equipment for lattice parameter, relative intensity, and other parameters critical for accurate measurements. Major instrument vendors provide this SRM to certify the performance of their equipment. SRM 1976a is the successor to SRM 1976 and, in response to customer feedback, was manufactured and measured to tighter tolerances than the original SRM.