This paper describes a new approach for quantification of rates of molecular transport through patterned, or otherwise heterogeneous, porous films supported on conductive substrates. Scanning Electrochemical Microscopy (SECM) has been used to image molecular sieving of redox active probes by thin, electropolymerized films of Fe(5-amino-1,10-phenanthroline)32+ on micropatterned and microdisk array electrodes. Films as thin as 12 nm completely block redox mediators with average molecular diameters greater than 12 Å, whereas smaller diameter probes (radii 5-8 Å) were observed to permeate selectively. SECM tip currents measured for three different redox permeants/mediators are observed to decrease with increasing polymer thickness, consistent with a transport model that includes partitioning into and diffusion within the polymer films. Permeabilities, PD(f), within the poly[Fe(5-NH2-phen)32+] films have been quantitatively determined from the SECM tip currents and are in excellent agreement with data previously obtained from rotating- disk electrochemistry. This new methodology provides a versatile approach for quantitative investigation of membrane transport and permeation selectivity with good lateral spatial resolution.