High-resolution X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy were applied to characterize GaAs(100) surface engineered by self-assembled monolayers (SAMs) of 4'-substituted aromatic molecules: 4'-methyl-4-mercaptobiphenyl (CH3-BPT) and 4'-hydroxy-4-mercaptobiphenyl (OH-BPT). Both of these molecules formed ordered and densely packed SAMs on GaAs, which were able to protect the substrate from degradation under ambient conditions. The molecular attachment in the SAMs is mediated by As-thiolate bond while the intact aromatic backbones have an upright orientation with average tilt angles of 31.0degrees and 37.2degrees for CH3-BPT and OH-BPT films, respectively. The difference in the tilt angle is attributed to a higher (by 7-10%) packing density of the former SAM, suggesting that the character of 4'-substitution affects the quality of the resulting SAM on the GaAs substrate.