The adsorption and supramolecular ordering of trimesic acid (TMA), 1,3,5-benzenetricarboxylic acid C6H3(COOH)(3), on a Cu(100) surface has been studied in-situ in ultrahigh vacuum by variable-temperature scanning `tunneling microscopy. We have elucidated the real-space structures of distinct self-assembled supramolecular aggregates at the molecular scale. At low temperatures (similar to200 K), two-dimensional networks evolve. They reflect extensive hydrogen-bond formation of a flat-lying species, similar to supramolecular ordering in the bulk TMA crystal structure. At room temperature, more densely packed stripe arrangements form. They are associated with a bonding transition leading to an upright geometry due to carboxylate formation.