Octadecanethiol (ODT), CH3(CH2)17SH, self-assembled monolayers (SAMs) were deposited on GaAs (100) substrates from solution and from vapor phase. The monolayers assembled from solution were prepared by incubating the substrate in a dilute millimolar ODT/ethanol solution, while vapor-deposited monolayers were prepared from vapor phase transport of ODT in an ultrahigh vacuum environment. The structural and optical characteristics of the resulting SAMs were examined with contact angle (CA) analysis, photoluminescence (PL) spectroscopy, atomic force microscopy (AFM), and spectroscopic ellipsometry. Static CA measurements of the SAMs indicated the successful formation of highly organized monolayers by both preparation routes. In contrast, PL spectroscopy, AFM, and ellipsometry measurements showed differences in the optical and structural properties of SAMs prepared from liquid and vapor phases. The data obtained indicate that more densely packed monolayers exhibiting greater surface coverage and therefore, increased passivation characteristics are assembled from vapor phase.