A series of reinforced composite membranes were prepared from Nafion (R) 212 and crosslinkable fluorine-containing polyimide (FPI) with various crosslinkers The crosslinkable FPI reacts with the crosslinkers and forms semi-interpenetrating polymer networks (semi-IPN) structure with Nafion (R) 212 The water uptake swelling ratio mechanical properties thermal behavior proton conductivity and chemical oxidation stability of the composite membranes are studied The degree of crosslinking is characterized by gel fraction of the composite membranes Compared to pure Nafion (R) 212 the composite membranes exhibit excellent thermal stability Improved mechanical properties and dimensional stability The tensile strength of the composite membranes is in the range of 37 3-51 2 MPa All the composite membranes exhibit high proton conductivity which ranges from 1 9 x 10(-2) to 99 x 10(-2) S cm(-1) The proton conductivity of the composite membrane with 2-propene-1-sulfonic acid sodium salt (SAS) as the crosslinker is 99 x 10(-2) S cm(-1) at 100 degrees C which is similar to that of Nafion (R) 212 under the same condition (C) 2010 Elsevier Ltd All rights reserved