Herein, mixed matrix membranes including alumina particles whose surfaces have been differently modified have been made and characterized. The polymeric matrix is a fluorinated polyimide, 6FDA-6FpDA, whereas the inorganic constituent parts are neat, silanized or poly-vinyl-pirrolidone-coated nanometric gamma-Al2O3 particles of 40 nm in diameter, which have been added in an approximated percentage of a 12% w/w. All the modified or unmodified added particles lead to similar improvements in the permselectivity of the gas pairs studied. Only in the case of the O-2/N-2 pair the increase in selectivity for the silanized particles is not as effective as the observed permeability enhancement for the uncoated and PVP-coated particles. In case of CO2/CH4, the 1991 Robeson's upper bond line is overpassed. The characteristics of the permselectivity improvement for the different inorganic particles have been analyzed in terms of the properties of the particle-polymer interfacial properties. (C) 2009 Elsevier Ltd. All rights reserved.