The electrochemically measured sieving and transport properties for two new thin film materials composed of molecular squares featuring exceptionally large cavities, a salen-based zinc square 2 (23 Angstrom metal to metal separation distance) and a porphyrin-based zinc square 1 (24 Angstrom metal to metal separation distance) are described. Both materials are exceptionally porous with respect to smaller molecules but blocking toward larger permeants (i.e., those larger than the square cavities). Electrochemical experiments reveal that the size cutoff for molecular sieving can be modified by incorporating difunctional ligands within the square cavity responsible for the observed porosity. (C) 2002 The Electrochemical Society.