Self-assembled monolayers of several alkanethiol molecules, with varying chain length and terminal groups, were investigated using x-ray photoelectron spectroscopy (XPS). The chain lengths varied from 10 to 15 methylene units, and the terminal group was either hydrophilic carboxylic acid and hydroxyl or hydrophobic methyl. The alkanethiol molecules were deposited on GaAs surfaces from liquid solutions. The impact of atmospheric exposure was examined by investigating one set of samples stored under atmospheric conditions and a second set that was stored in a nitrogen atmosphere prior to analysis. Carbon, oxygen, gallium, and arsenic core level XPS spectra were obtained on all surfaces. The intensity of the gallium and arsenic core levels indicates a considerable difference in the Ga/As ratio dependent on the terminal group of the alkanethiol. Additionally, the carbon and oxygen spectra indicate varying chemical bonding on the surface with the alkanethiol's having a carboxylic acid terminal group showing a more complex carbon and oxygen bonding. (c) 2006 American Vacuum Society.