Internal concentration polarization (ICP) in osmotic processes is largely influenced by the porous structure of the support layer of the membrane. Recent publications on osmotic separations have described a 'structural parameter', S (function of the thickness, tortuosity, and porosity of the support layer), that represents the support layer's contribution to the overall mass transfer resistance during osmosis. To date, S has only been calculated as a fitted parameter in a model that requires experimental flux measurements. Such a method is inaccurate since the models tail to account for all of the different mass transfer phenomena. An alternative is to characterize the thickness, tortuosity, and porosity independently and thus calculate the actual value of the structural parameter. However, for soft materials like porous membranes, no standard methods have been established for measuring porosity and tortuosity. In this study, we propose the use of X-ray microscopy (XRM) for determining the structural parameter of thin film composite (TFC) membrane support layers. The S value could be calculated from the XRM images and was compared to the results obtained from more conventional mercury intrusion porosimetry as well as an existing model used with empirical data. Substantial differences between the values obtained from the different techniques indicated the need to revise the traditional approaches of characterizing membrane structures. (C) 2013 Elsevier B.V. All rights reserved.