Poly(amidoamine) (PAMAM) dendrimer composite membranes with high CO 2 /N 2 selectivity and CO 2 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 CO 2 separation. Permeation experiments of the PAMAM dendrimer composite membrane conducted in a pencil module (an effective membrane area: ca. 17cm 2 ) were carried out using a mixed CO 2 (5%)/N 2 (95%) feed gas at a pressure difference of 97kPa at 40°C. The PAMAM composite membrane exhibited an excellent CO 2 /N 2 selectivity of 230 and a CO 2 permeance of 4.6×10 −7 m 3 (STP)m −2 s −1 kPa −1 (=61GPU).SEM observations of the composite membrane revealed a thin chitosan layer (ca. 200nm 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. 300nm thick) suitable for CO 2 separation.