Self-assembled monolayers of 4-mercaptobenzoic acid exhibit infrared spectral features for the carboxylic acid functionality that are characteristic of isolated vapor-phase molecules (e.g., sharp O-H stretch band, high-frequency non-hydrogen-bonded carbonyl stretch band) and inconsistent with molecules that are dimerized or that reside in condensed-phase hydrogen-bonding environments. This is in sharp contrast with the behavior of omega-mercaptoalkanoic acid monolayers, which exhibit spectral features characteristic of hydrogen-bonded and/or dimerized acid groups (e.g., no obvious O-H stretch band, lower frequency carbonyl stretch band). It is postulated that these characteristic differences reflect differing degrees of conformational rigidity in the monolayers, with the aromatic monolayers being sufficiently rigid and oriented as to prevent the intermolecular hydrogen bonding/dimerization that is commonly observed in aliphatic monolayers.