Scanning tunnelling microscopy is used to analyze the structure of two-dimensional polycrystalline adlayers of 2-pyridone-5-carboxylic-acid-[4’-(n-octyloxy)phenyl]-ester on the basal plane of graphite. The observed structure can be explained in terms of a hydrogen-bridged adlayer of close-packed dimerised molecules. Real-time imaging on a length scale of nanometres indicates a non-directed dynamical process at domain boundaries of neighbouring crystallites of similar size.