We compare the interfacial reactivity between vapor- or liquid-phase heptanoyl chloride (C6COCl) and hydroxyl-terminated monolayers prepared from 4-hydroxythiophenol (HTP), 11-mercaptoundecanol (MUD), and a hydroxyl-terminated, fourth-generation poly(amidoamine) (PAMAM) dendrimer (G4-OH). Fourier transform infrared-external reflection spectroscopy indicates that both vapor- and liquid-phase C6COCl reacts with all three hydroxyl-terminated self-assembled monolayers (SAMs) to yield ester-coupled bilayers. The spectroscopic data were confirmed by contact-angle goniometry, which revealed advancing contact-angle increases of between 60 and 75 degrees after vapor-phase reaction, and ellipsometric measurements, which indicated that the films increased in thickness by 0.4-0.6 nm. Similar results were obtained when the ester-coupling reaction was carried out in liquid-phase CH2Cl2 Real-time, quantitative surface acoustic wave (SAW)-based nanogravimetry indicates that the vapor-phase reactions go to completion in < 1 min. The order of reactivity of the monolayers decreases in the order G4-OH > MUD > HTP. This is interpreted in terms of monolayer structure and the intrinsic properties of the particular coupling reaction studied.