Thin polymeric membranes have been formed by liquid/liquid interfacial copolymerization of a sterically demanding tetraphenylporphyrin derivative having reactive phenol substiments and a second porphyrin having reactive acid chloride groups. The out-of-plane steric demand is created by 3,5-hexoxyphenyl groups positioned at two of the four meso carbons of the porphyrin ring. The bulky substituents were designed to create local pockets and extended pores within the resulting ester-linked copolymer. Quantitative measures of molecular and ionic transport were obtained by placing membranes over microelectrodes and recording voltammetric responses from redox-active probes. The membranes were found to be permeable to small molecules and ions, but blocking toward larger ones, displaying a sharp size cutoff at a probe diameter of ca. 3.5 A. Molecular transport can be modulated by axially ligating pore-blocking moieties to available porphyrin metal centers.