This study investigated the use of nanosized fumed silica (Aerosil (R) R106) as fillers in the production of asymmetric hollow fiber mixed matrix membrane (HFMMM) where polysulfone was the host polymer matrix. The presence of fumed silica particles were found to stimulate the flow of CO2 as indicated by an increase (>12%) in CO2 permeability for all HFMMMs. At low loading of 0.1% (w/w), the permeability of CO2 and CH4 were 90.04 and 2.75 GPU, respectively, and the calculated selectivity was higher than the neat PSf hollow fiber membrane. At higher loading of 10% (w/w), the FESEM images of the HFMMM showed severe agglomeration of fillers that contributed to nanometric defects between the aggregates, leading to higher permeation of slow gases (CH4 and N-2). Hence, the selectivities of CO2/CH4 and O-2/N-2 were low at 7.43 and 2.02, respectively. The calculation of particle spacing within the polymer macromolecules have shown that for nanoparticles, the agglomeration was unavoidable at high loading due to sheer number of particles within the matrix. At low loading of 0.1%, the polysulfone-fumed silica HFMMM showed no visible sign of particle agglomeration. The selectivities of CO2/CH4 and O-2/N-2 significantly improved with an average value of 32.74 and 6.35, respectively. The HFMMM also showed an increase in glass transition temperature and has a better thermal stability as measured by DSC and TGA analyzers, respectively. The importance of filler-filler, polymer-filler and filler-gas interactions on the characteristic and permeation performance of HFMMMs are also discussed. (C) 2011 Elsevier B.V. All rights reserved.