Understanding the interactions between molecules on surfaces is crucial for advancing technologies in sensing, catalysis, and energy harvesting. In this study, we explore the complex surface chemistry resulting from the interaction of Co(II)octaethylporphyrin (CoOEP) and iodine, I2, both in solution and at the phenyloctane/highly oriented pyrolytic graphite (HOPG) interface. In pursuit of this goal, we report results from electrochemistry, NMR and UV−vis spectroscopy, X-ray crystallography, scanning tunneling microscopy (STM), and density functional theory (DFT). Both spectroscopic methods of analysis confirmed that at, and abov,e the stoichiometric ratio of one CoOEP to one I2 the reaction product was metal-centered CoIII(OEP)I. Xray crystallography verified that a single iodine is bonded to each cobalt ion in the triclinic P1̅system. The surface chemistry of CoOEP and I2 is complicated and remarkably dependent on the iodine concentration.