The Si 1s (K-shell) x-ray absorption spectra of several [(Si)m(Ge)n]p atomic layer superlattices (ALS) and a range of SixGe1−x alloy thin films grown epitaxially on both Si(100) and Ge(100) have been investigated using plane-polarized synchrotron radiation. The near-edge spectral features of ALS and alloy samples with similar (average) chemical composition are remarkably similar. The spectra of both the strained ALS and alloy samples contain features at the Si 1s threshold which exhibit a small but characteristic polarization dependence. The polarization dependence is reduced or absent in strain-relaxed materials such as annealed ALS, annealed alloys, or thick alloy films. The polarization-dependent components of the signal are attributed to anisotropic states associated with strain-induced tetragonal distortions. The sense of the polarization is inverted between samples grown on Si and those grown on Ge, consistent with the expected inversion in the spatial orientation of the strain field. An explanation is proposed for the dependence of the magnitudes of the Si 1s polarization effect on the composition of alloys and ALS.