A silicon powder Standard Reference Material, SRM‐640, has been prepared for use as a standard in powder diffractometry. Powder diffraction measurements were performed with a tungsten internal standard and a high‐angle goniometer. The measureda/λ is 3.525176. With λ(CuKα1peak) taken as 1.5405981 Å,a= 5.430880 (35) Å, uncorrected for refraction. Comparison of a with values obtained with a single‐crystal from one of the boules reveals a difference of 3 parts in 105. This difference suggests a subtle systematic error in powder diffractometry or a change in lattice spacing near crystal boundaries. Use of the SRM should permit individual measurements of lattice parameters to be made reproducible to near 1 part in 105and an absolute accuracy of at leas