The porosity is a key characteristic parameter and significantly influences the behavior of a membrane in a given application. This work demonstrates the use of infrared (IR) spectroscopy for mapping the porosity of polymer membranes. Conventional transmission IR spectroscopy is employed to assess the bulk porosity across the entire thickness of a polyethersulfone (PES) membrane, while attenuated total reflection IR (ATR-IR) provides information about the porosity of a thin layer at the membrane surface. The results indicate that the bulk porosity of the studied membrane is rather homogeneous while the surface porosity is characterized by systematic local deviations from its mean value up to about 30%. IR spectroscopy turns out to be a suitable and straightforward method to characterize the porosity distribution of membranes. (C) 2013 Elsevier B.V. All rights reserved.