Visible spectroscopy and electrochemistry have been employed to study monolayer films of 5,10,15,- 20-tetrakis[o-(2-mercaptoethoxy)phenyl]pin free base, H-2(o-TMEPP), and cobalt metalated Co(o-TMEPP) forms chemisorbed onto transparent gold electrodes. Transmission spectra of these films are consistent with approximately monolayer((4.8-6.5) x 10-(11)mol/cm(2)) coverages and edge-edge porphyrin-porphyrin interactions within the monolayers as revealed by red shifts and broadening in the Soret bands compared to solution species. Chemical modifications of H-2(o-TMEPP) monolayers namely, cobalt metalation and protonation/deprotonation, are described. The pK(a) of the free base porphyrin monolayer is ca. 2.5. Spectroelectrochemical experiments of Co(o-TMEPP) monolayers allow optical tracking of the redox state of the porphyrin, which was applied in electrocatalytic dioxygen reduction to show that the onset of dioxygen reduction is coincident with the Co(III) --> Co(II) process in the monolayer and that the lower limit of the two-electron catalysis rate constant is 6 x 10(4) M(-1) s(-1).