Stoichiometric GaAs(100) surfaces were functionalized with monolayers of an aromatic compound, 1,1'-biphenyl-4-thiol (BPT), and the engineered surfaces were studied by synchrotron-based high-resolution X-ray photoelectron spectroscopy and near edge X-ray absorption fine structure spectroscopy. BPT molecules were found to form a well-ordered and densely packed self-assembled monolayer on these substrates. The attachment to the substrate occurs over the thiolate headgroup while the intact biphenyl moieties have an upright orientation with an average tilt angle of 31.5degrees +/- 5degrees. The functionalization of GaAs by BPT was found to prevent an oxidation and contamination of the substrate, keeping the GaAs surface in a pristine state. The surface engineering of GaAs with functionalized aromatic monolayers can provide a crucial link for combining GaAs-based semiconductor nanostructures with bio-organic molecular assemblies.