We used Fourier transform infrared external reflectance spectroscopy (FTIR-ERS), nanogravimetry based on thickness-shear-mode resonators (TSMRs), and X-ray photoelectron spectroscopy (XPS) to study self-assembled monolayers (SAMs) of three mercaptobenzoic acid (MBA) isomers on Au and their interactions with vapor-phase decylamine. FTIR-ERS spectra of the 4-, 3-, and 2-MBA SAMs indicate that the proximity of the carboxylic acid group to the Au substrate surface affects the electronic environment of the benzene ring and the acidity of the carboxyl-group proton. TSMR results show that reaction of 4-, 3-, and 2-MBA monolayers with vapor-phase decylamine probe molecules results in decylamine fractional surface coverages of 0.97, 0.83, and 0.45 of the theoretical monolayer maximum, respectively. The gravimetric results are corroborated by in-situ FTIR-ERS difference spectra of the MBA monolayers obtained during reaction with decylamine, which show the simultaneous disappearance of carbonyl and hydroxyl bands and appearance of carboxylate and aliphatic hydrocarbon bands. XPS results show that increasing the proximity of the carboxyl group to the surface results in a relative increase in the proportion of carboxyl-group oxygen that is in the 531.0 eV electron binding energy state compared to the 532.5 eV state, indicating a change of chemical environment for oxygen. The overall results are consistent with a model involving proton transfer from the SAM to the vapor-phase bases wherein acid strength depends on the accessibility of the donor group, and they demonstrate a clear structure/reactivity correlation for surface-confined isomers, which illustrates the dramatic constraints imposed by surface-induced ordering compared to anisotropic bulk-phase reactivity.