Abstract The geometric structure of an ordered monolayer of large organic molecules adsorbed on a metallic surface [thiouracil on Ag(111)] was analyzed using surface X-ray diffraction. As in the bulk crystal structure, the surface adsorbed molecules are arranged in parallel chains. However, owing to the interaction of the S atoms with the substrate as well as to the reduced dimensionality and lower lateral packing density, all but one of the hydrogen bonds characteristic of the bulk structure are broken. This causes a doubling of the chain repeat periodicity. The intact pyrimidine rings are tilted by 31 and 36° with respect to the Ag surface. In addition, an elongation of the CS bond distance by 0.17 Å and an increase of the intra-pyrimidine-ring distances by about 0.10 Å relative to the bulk are observed.