Self-assembled monolayers of nickel- and vanadyloctaethylporphyrin molecules (NiOEP and VO-OEP, respectively) formed oil bare and on 5-(octadecyloxy)isophthalic acid (5-OlA) covered highly ordered pyrolytic graphite (HOPG) substrates were studied with scanning tunneling microscopy (STM) at the solid-liquid interface under ambient conditions. A detailed comparison of the monolayer structures and lattice parameters of Ni-OEP and VO-OEP overlayers, along with previous information about the structure of Pt-OEP monolayers, suggests that coupling between the central metal atom and the substrate strongly affect the observed structures. In addition, the concentration of the solution and the nature of the solvent also affect the structure of these thin porphyrin films. These conditions call be used to guide the nanoscaled structures that form.