Positron annihilation spectroscopy coupled with a slow positron beam was used to characterize the depth profile of the cavity size in situ in thin-film-composite polyamide (PA) nanofiltration membranes fouled in calcium sulfate feed solutions with different pH values. Systematic study of the performance of membranes used for filtering reasonably acidic feed solutions showed that after cleaning the fouling layer from the membranes' surface, pure water flux increased significantly and the rejection of magnesium ions decreased significantly. A slow positron beam with a positron incident energy up to 30 key was adopted to determine the Doppler broadening R parameter and S parameter, ortho-Positronium (o-Ps) lifetime and o-Ps intensity, which correspond to the layer structure, cavity size and cavity density of the composite membrane, respectively. A change in the thickness of the PA layer was also confirmed by R parameter analysis and field-emission scanning electron microscopy. The o-Ps lifetime and cavity size distribution in the membranes indicate that monovalent ions permeate through the membranes, leaving behind divalent ions largely retained by the selective NF membranes. (C) 2011 Elsevier B.V. All rights reserved.