The x‐ray standing wave (XSW) technique was applied to determine the lattice location of Si impurity atoms in GaAs(100) crystals. The synchrotron radiation of X24A at the national synchrotron light source was utilized to set up backreflection XSW, an experimental geometry which drastically relaxes the otherwise stringent requirement on the lattice perfection. Specifically, the lattice sites were determined with respect to the [311] reflection planes which differentiate a Ga site from an As site. With the aid of an appropriate choice of the x‐ray fluorescence filter, we were able to study GaAs(100) samples with very low levels of Si impurities. On a sample doped with 4×1018cm−3Si during the molecular‐beam epitaxy growth, we found that the Si atoms predominantly occupied the Ga sites. On both an ion‐implanted sample after annealing and a sample with Si impurities introduced by thermal diffusion, about 30% of the Si atoms occupied the Ga sites, and the rest occupied random sites. The As site occupation was less than 6%. Suggestions are made for further experiments with improved sensitivity.