The initial stages of interface formation for In and Sb, each separately adsorbed on the Ge(100)-(2×1) surface, have been investigated for various submonolayer coverages with use of synchrotron-photoemission and high-energy electron-diffraction techniques. By examining the evolution of the substrate and adsorbate core-level line shapes during the adsorbate growth it is possible to deduce chemical and structural information. The dimer-related surface-shifted component of the Ge 3d core level is found to gradually convert into a component possessing a binding energy indistinguishable from the bulk component for increasing Sb coverages. For submonolayer In depositions, this surface-shifted component results in a chemically shifted interface component which is distinguishable from the emission from bulk atoms and unreacted Ge dimers. The resulting chemical shifts are consistent with expectations based on electronegativity arguments. For both In and Sb, the adsorbate-to-substrate bonding coordination number is obtained for various submonolayer coverages. The homogeneity of the adsorbate bonding is evaluated by examining the width of the In and Sb 4d core-level line shapes. The Fermi-level position relative to the gap for various In and Sb coverages and the Schottky-barrier heights are obtained. Photoemission from the valence bands shows a modification in the dimer-derived surface states upon In and Sb adsorption; the surfaces show little density of states at the Fermi level for coverages below 1 monolayer. Structural models for the In/Ge(100)-(2×2) and Sb/Ge(100)-(2×1) surfaces which correlate with the data are discussed.