Free-standing ultrathin films of a porphyrin homopolymer have been synthesized by interfacial polymerization of a mercaptoporphyrin. Coupling of 5,10,15,20-tetrakis(alpha-mercapto-p-tolyl)porphyrin at the water/chloroform interface by copper acetate yields continuous, flexible films after drying with supercritical CO2. These films are amorphous and retain many characteristics of the monomeric porphyrin, such as the property that they may be metalated. Spectroscopic evidence suggests efficient cross-linking of thiols through disulfide coupling occurs in these polyporphyrins to form a network. Polymerization is accompanied by the introduction of carbonyl functionality, probably the transformation of residual thiols to thioacetate by metal-mediated reaction with excess acetate ion. As an example of the catalytic properties of these polymers, they are shown to promote photooxidation of thietane to the corresponding sulfoxide by sensitizing singlet oxygen production.