The selectivity, sensitivity, and reproducibility of SAW chemical sensors functionalized with a carboxylate-coordinated Cu2+-terminated surface are examined in relation to the interfacial properties of these organomercaptan self-assembled monolayer (SAM) films, prepared from the solution phase for adsorption times of 36, 84, or 180 h on Au surfaces having variable, controlled grain sizes. SAM adsorption time and the grain size of the Au film can dramatically affect the response to adsorption from the vapor phase onto the composite monolayer-modified SAW device. It is proposed that the varied response results from differences in molecular packing and, particularly, in the ordering of the end groups of the monolayer film that provide the chemically sensitive interface. These studies provide an important step toward reliably fabricating chemical sensors that respond to specific classes of organic analytes.