The adsorption and thermal decomposition of acetic acid and deuterated acetic acid on clean Ru(0001) were studied by reflection absorption infrared spectroscopy. Acetic acid adsorbs molecularly, with dimer formation, at 100K. At 113K, the molecular form is still predominant, although some dissociative adsorption already occurs. Dehydrogenation is complete at 123K, leaving adsorbed acetate as a symmetric bidentate species. Depending on coverage, two bonding configurations are proposed: a bidentate bridging [μ 2 -η 2 (O,O)-CH 3 COO], with each oxygen bonded to a neighbour Ru atom, at low coverages, and a bidentate chelating [η 2 (O,O)-CH 3 COO], with the two oxygen atoms bonded to the same Ru, at higher coverages. The decomposition temperature increases with coverage, giving evidence in favour of a stabilizing effect of acetate ions and decomposition products on neighbour adsorbed acetates. For high enough coverages, adsorbed acetate was still detected at 500K, proving the high stability of this surface species on Ru(0001). The adsorption of deuterated acetic acid gave consistent results, although dissociative adsorption starts occurring at 123K and complete dehydrogenation occurs at 133K, ~10K above the respective temperatures for acetic acid.