Oxidative electropolymerization at glassy carbon and indium-tin-oxide electrodes from unbuffered aqueous solutions of Fe(5-NH2-phen)(3)(2+) and Ru(5-NH2-phen)(3)(2+) yields electrode-supported films that self-limit at thicknesses of ca. 50 and 2 nm, respectively. Electrodeposition is characterized by formation of nonelectroactive films, which are electronically insulating at least over the electrochemical potential range of 0 to +1 V. Electrochemical measurements of redox-active probe molecules demonstrate that these films exhibit highly selective molecular sieving. Neutral permeants and hydrophobic permeants are observed to penetrate the polymeric coating more rapidly than charged or hydrophilic forms. Atomic force microscopy, UV-vis spectroscopy, and X-ray photoelectron spectroscopy were used to correlate the polymeric molecular structure with the observed electrochemical reactivity. Molecular sieving behavior is discussed in terms of a membrane transport model implying that the films are comprised of metallopolymeric molecular cavities of uniform size and distribution.