Matrimid and 6FDA-durene polyimides (PI) are chosen to investigate diamino cross-linking processes in solution and dense membrane forms. The cross-linking process (precipitation) of polyimide solutions is visually monitored by a digital camera to provide visual evidence for the chemical reaction. By comparing the reaction of different diamines with the polyimide solutions, we inferred that the cross-linking rate is an indication of the nucleophilicities of the diamines. The cross-linked network in Matrimid/dichloromethane (CH2Cl2) solutions appeared much faster than that in 6FDA-durene/CH2Cl2 solutions. Interestingly, the cross-linking of 6FDA-durene membranes by the same diamine resulted in a greater decline in as permeability as compared to that of Matrimid membranes due to the different electrophilicity of polyimides and the different diffusion rate of diamines into membranes. FTIR, XPS, XRD, TGA and gas transportation test are applied for analyzing the cross-linking mechanism and properties of polyimide membranes. Gas pair selectivities of H-2/CO2, H-2/N-2, O-2/N-2 and CO2/CH4 were enhanced upon the cross-linking of 6FDA-durene by diamines. In contrast, for Matrimid, an improvement in gas pair selectivities of H-2/CO2, H-2/N-2 and O-2/N-2 was achieved but a less favorable CO2/CH4 selectivity was observed after diamino cross-linking. (C) 2008 Elsevier B.V. All rights reserved.