X-ray photoelectron spectroscopy is used to measure the valence-band offset in situ for GaAs/AlAs (100) heterojunctions grown by molecular beam epitaxy. Ga 3d and Al 2p core level to valence-band edge binding energy differences are measured in GaAs (100) and AlAs (100) samples, respectively, and the Al 2p to Ga 3d core level binding energy difference is measured in GaAs–AlAs (100) and AlAs–GaAs (100) heterojunctions. Measurements of the Al 2p to Ga 3d core level energy separations indicate that the band offset for GaAs/AlAs (100) is commutative; the value we obtain is ΔEv=0.46±0.07 eV. Our observation of commutativity is believed to be a consequence of the high quality of our GaAs/AlAs (100) heterojunctions, and of the inherent commutativity of the GaAs/AlAs (100) band offset.