Poly(amidoamine) (PAMAM) dendrimer composite membranes with high CO2/N-2 selectivity and CO2 permeance were developed in situ according to the in situ modification (IM) method. This method utilizes the interfacial precipitation of membrane materials on the surface of porous commercially available, polysulfone ultra-filtration hollow fiber membrane (molecular weight cutoff: 6000) substrates. A thin layer of amphiphilic chitosan, which has a potential affinity for both hydrophobic PSF-substrates and hydrophilic PAMAM dendrimers, was employed as a gutter layer directly beneath the inner surface of the substrate by the IM method. PAMAM dendrimers were then impregnated into the chitosan gutter layer to form a hybrid active layer for CO2 separation. Permeation experiments of the PAMAM dendrimer composite membrane conducted in a pencil module (an effective membrane area: ca. 17 cm(2)) were carried out using a mixed CO2 (5%)/N-2 (95%) feed gas at a pressure difference of 97 kPa at 40 degrees C. The PAMAM composite membrane exhibited an excellent CO2/N-2 selectivity of 230 and a CO2 permeance of 4.6 x 10(-7) m(3) (STP) m(-2) s(-1) kPa(-1) (=61 GPU). SEM observations of the composite membrane revealed a thin chitosan layer (ca. 200 nm thick) directly beneath the surface of the porous substrate. Hybridization of the PAMAM dendrimer with a chitosan gutter layer afforded a defect free active layer (ca. 300 nm thick) suitable for CO2 separation. (c) 2006 Elsevier B.V. All fights reserved.