The physical examination of pore size and nanostructure of nanofiltration (NF) membranes by imaging has proved elusive. In particular, until recently, no technique has been developed to image nanopores with dimensions in the 0.5-2 nm range. These pores are crucial for the functional performance of NF membranes, and thus characterisation of their architecture might lead to tailored design of membranes for specific separation processes. This progress might be achieved via improvement in understanding the membrane structure described at the macromolecular level. Electron microscopy imaging of nanoporous polymers at sub-nanometre resolution is difficult due to insufficient electron contrast of polymers. One new approach to improving the characterisation methods is the use of high electron contrast osmium dioxide (OsO2) nanoparticles (NF), which are lodged in situ into the membrane pores. By mapping these nanoprobes using transmission electron microscopy (TEM), polymer nanostructures can be imaged at magnifications near the macromolecular level. In this paper, organic solvent nanofiltration (OSN) membranes with thin film composite (TFC) structures were characterised in terms of the permeation pathway ('pore') size and separation layer morphology. The estimated pore size correlates well with the solute rejection, implying that pore-flow models may be useful for describing the transport through the membranes. (C) 2013 Elsevier B.V. All rights reserved.