The commonly used Anderson electron affinity rule is shownnotto provide the band‐edge offsets at the interface between different semiconductors. Using synchrotron radiation excited photoelectron spectroscopy, we determine electron affinities &khgr; of 4.14+0.17−0.09 eV for a Ge(110) surface and of 4.15+0.17−0.09 eV for a 18‐A˚ GaAs(110) epitaxial overlayer on Ge(110). In the same experiment, for the same layers, a conduction‐band discontinuity &Dgr;Ecof 0.54±0.08 eV is measured for the heterojunction of GaAs grown by molecular beam epitaxy on Ge(110). Compilation of data on 14 recent photoemission studies confirms that &Dgr;Ec≠&Dgr; &khgr; for most heterojunction systems investigated to date.