We have used x‐ray photoelectron spectroscopy to measure the valence‐band offsetinsitufor strained Si/Ge (100) heterojunctions grown by molecular beam epitaxy. Si 2pand Ge 3dcore level to valence‐band‐edge binding energies and Si 2pto Ge 3dcore level energy separations were measured as functions of strain, and strain configurations in all samples were determined using x‐ray diffraction. Our measurements yield valence‐band offset values of 0.83±0.11 eV and 0.22±0.13 eV for Ge on Si (100) and Si on Ge (100), respectively. If we assume that the offset between the weighted averages of the light hole, heavy hole, and spin‐orbit valence bands in Si and Ge is independent of strain, we obtain a discontinuity in the average valence‐band edge of 0.49±0.13 eV.