Nafion(R) is a commercially available perfluorosulphonate cation exchange membrane commonly used as a perm-selective separator in chlor-alkali electrolysers and as the electrolyte in solid polymer fuel cells. This usage arises because of its high mechanical, thermal and chemical stability coupled with its high conductivity and ionic selectivity, which depend strongly on the water content. The membrane was therefore studied in different states of hydration obtained by placing the membrane and atomic force microscope (AFM) in a specially constructed environmental chamber to control the humidity. Tapping mode phase imaging was successfully used to identify the hydrophobic and hydrophilic regions of Nafion, The images support a cluster model for the hydrophilic regions of Nafion at humidities of (9-34) +/- 2%. The clusters have a range of sizes from 5 to 30 nm, significantly larger than the similar to 4 nm structures proposed from X-ray studies, which is probably due to the formation of cluster agglomerates. Phase images were interpreted semi-quantitatively in terms of energy loss, typically 3 kJ m(-2), and the number and size of energy dissipative features. The number of clusters decreased while the average cluster size increased with increasing humidity. (C) 2000 Elsevier Science Ltd. All rights reserved.