Mixed self-assembled monolayers (mixed SAMs) were formed by competitive adsorption of tert-butyl mercaptan and n-octadecyl mercaptan from 1 mM ethanol and toluene solutions onto planar gold substrates. The solution concentration ratio (R(soln)) of the competing adsorbates was varied from zero to infinity in order to produce surfaces with a range of adsorbate compositions and to determine the relationship between R(soln) and the surface concentration ratio of the adsorbed species (R(SAM)). The mixed SAMs were studied by contact angle goniometry, optical ellipsometry, and negative time-of-flight secondary ion mass spectrometry (TOF-SIMS). It was found that relative TOF-SIMS ion peak intensities may be used to determine the surface molecular composition of individual mixed SAMs within a related series. The TOF-SIMS data also revealed that the propensity for adsorbate oxidation increased with the concentration of the tert-butyl mercaptide adsorbate within the SAMs. Each of the surface analytical methods indicated that n-octadecyl mercaptan adsorbed onto gold with greater efficiency than tert-butyl mercaptan, by a factor of 65-160 from toluene and a factor of 290-710 from ethanol. The contact angle and thickness analyses provided evidence for physisorbed n-octadecyl mercaptan on surfaces prepared in ethanol that contained low surface concentrations of tert-butyl mercaptide (R(SAM) < 1, R(soln) = 10-100), though this could not be confirmed by negative TOF-SIMS.