A long-standing goal of materials microscopy is the imaging of nanoporous polymeric membranes at nanometer resolution. In particular, no technique has been developed to "see" nanopores with dimensions in the 0.5-2 nm range. These pores are crucial in the functional performance of nanofiltration (NF) membranes used for molecular separation processes. The major problem in imaging at this scale is insufficient electron contrast of polymers, which has made distinguishing their structural phases at the nanoscale not practically feasible. Here we describe an in situ physicochemical characterization technique to measure size of the transport active pores. Nanopores in an asymmetric organic solvent nanofiltration (OSN) membrane are filled with high contrast osmium dioxide (OsO2) nanoparticles, whose spatial arrangement is mapped under transmission electron microscopy (TEM). Due to the uniquely small size range of these probes, they target nanopores in the membrane skin layer. Thus, pore size distribution and even polymer chain structure can be evaluated. The pore size is found to correlate well with membrane separation performance, and the molecular separation mechanism of the nanofiltration P84 co-polyimide membrane is shown to be due to size exclusion of the larger nanoparticles. (C) 2012 Elsevier B.V. All rights reserved.