In ultrafiltration membranes, selective layer composed of nodular microstructure usually formed by nucleation and growth during phase separation. The nodule size and extent of nodule packing can be varied by the use of inorganic nanoparticles of zeolite to render hydrophilic and hydrophobic microdomain structure to the membrane which minimizes fouling. The zeolite nanoparticles are dispersed from 0.01 to 1 wt% in N-methyl-2-pyrrolidone (NMP) solvent with the compatibilizer (D-alpha-tocopheryl polyethylene glycol succinate, TPGS) to form nanocomposites of Psf/zeolite in the form of hollow fiber membranes (HFMs). High Resolution Scanning Electron Microscopy (HRSEM) and EDAX studies show that zeolite nanoparticles participate in the nucleation process during phase separation. An almost linear increase in the tensile modulus with nanoparticle concentration shows that the mechanical properties of the HFMs also get influenced. We observed that water permeability of HFMs increases from 15.92 to 21.31 mL/m(2) h mm Hg, when zeolite loading increased from 0.01 to 0.1 wt% loading. Further, permeability decreases to 11.79 mL/m(2) h mm Hg at 1 wt%. The molecular weight cut off of composite HFM shows a steady increase with loading concentration from 9500 Da to 54,000 Da. We proposed a microstructural model explaining the influence of zeolite addition on HFM properties which forms the basis for selection and optimization of such additives. Crown Copyright (C) 2012 Published by Elsevier B.V. All rights reserved.