A series of reinforced composite membranes as proton exchange membranes were prepared from Nafion(R)212 and crosslinkable fluorine-containing polyirnides (FPI). FPI was prepared from the polymerization of 4,4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA), 2,2'-bis(trifluoromethyl)4,4'-diaminobiphenyl (TFMB), and 3,5-diaminobenzoic acid (DABA). Then FPI was thermally crosslinked during the membrane preparation and formed the semi-interpenetrating polymer networks (semi-IPN) structure in the composite membranes. The thermal properties of the composite membranes were characterized by thermogravimetric analysis. The crosslinking density of FPI in the composite membranes was evaluated by the gel fraction. These membranes showed excellent thermal stabilities and good oxidative stabilities. Compared with Nafion(R)212, the obtained composite membranes displayed much improved mechanical properties and dimensional stabilities. The tensile strength of the composite membranes was more than twice that of Nafion(R)212. The composite membranes exhibited high proton conductivity, which ranged from 2.3 x 10(-2) S cm(-1) to 9.1 x 10(-2) S cm(-1). All membranes showed an increase in proton conductivity with temperature elevation. (C) 2009 Elsevier B.V. All rights reserved.