The structure of molecular monolayers formed at the interface between atomically flat surfaces and a solution of free-base meso-tetradodecylporphyrins (H(2)Ps) was examined by scanning tunneling microscopy (STM) at the liquid/solid interface. On the surface of graphite (HOPG), H(2)Ps form a well-ordered monolayer characterized by an oblique unit cell. On Au(111), H(2)Ps form a self-organized monolayer comprised of two distinct domain types. In both types of domains, the density of the porphyrin cores is increased in comparison to the arrangement observed on HOPG. Also, high-resolution STM images reveal that, in contrast to what is observed on HOPG, physisorption on Au(111) induces a distortion of the porphyrin macrocycle out of planarity. By using X-ray photoelectron spectroscopy, we demonstrate that this is likely to be due to the coordination of the lone pairs of the iminic (-C=N-) nitrogen atoms of the porphyrin macrocycle to Au(111).