The interaction of acetic acid and acetaldehyde with preoxidized Ag{111} has been studied using reflection absorption infrared spectroscopy. A bidentate bridging acetate species which shows no coverage-dependent orientation can be formed either by direct deprotonation of acetic acid by surface atomic O at 240 K or via O-induced polymerization of acetaldehyde at 140 K, followed by decomposition of the polyacetaldehyde intermediate into ethane-1,1-dioxy fragments that dehydrogenate at 220 K. A symmetry analysis reveals that the vibrational spectra of polyacetaldehyde and paraformaldehyde on many metal surfaces can be best interpreted in terms of helical chain structures bound to the surface by skeletal O atoms located along various points of the polymer backbones.