Ultrathin S-containing polymer films of about 5 nm in thickness were deposited on the HCl-etched (100)oriented single-crystal GaAs substrates via RF plasma polymerization of bis(methylthio)methane (BMTM). The chemical composition and structure of the BMTM plasma-polymerized GaAs(100) surface (pp-BMTM-GaAs surface) were investigated by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS), respectively. The XPS and ToF-SIMS results showed that the sulfur atoms from the plasma-polymerized BMTM (pp-BMTM) film were bonded to both the Ga and As atoms. The low-temperature photoluminescence efficiency of the so-passivated GaAs single crystal was increased by 2-fold. The growth of the oxide layer on the pp-BMTM-GaAs surface was effectively hindered for up to at least 2 months under the atmospheric conditions. The rate of surface oxidation was also reduced significantly in the presence of the pp-BMTM barrier when the HCl-etched GaAs(100) was exposed to water and H2O2 solution. The 180degrees-peel adhesion test results indicated that the pp-BMTM film adhered strongly to the GaAs(100) surface.