Vibrational spectra from monolayer films of acrylic acid (CH2CHCOOH) on oxidized aluminum are obtained by inelastic electron tunneling spectroscopy and multiple reflection-absorption infrared spectroscopy. Analysis of these spectra shows that acrylic acid does not adsorb exclusively by a dissociative mechanism as do most carboxylic acids. Instead, at least two coadsorbed forms are observed : physisorbed molecular acid and chemisorbed acrylate ion. Rinsing the dosed surface with ethanol removes the physisorbed form while leaving undisturbed the chemisorbed form.