The interaction of acetaldehyde with the Ag{111}-(4 x 4)-Ag1.83O surface oxide structure at temperatures between 77 and 200 K has been studied with scanning tunneling microscopy at 5 K. The bonding sites, relative to the oxide structure, of acetaldehyde at 77 K, and of acetate and ethane-1, 1-dioxy formed after annealing to 170 K are discussed. Helical polyacetaldehyde chains formed at 140 K, previously proposed on the basis of RAIRS data, have been observed directly, coexisting with the oxide surface.