In this work, sulfonated poly(phthalazinone ether nitrile ketone)/boron phosphate (SPPENK/BPO4) composite membranes with BPO4 content from 10 to 30 wt.% were prepared from SPPENK matrix, tripropylborate (C3H7O)(3)B and phosphoric acid (H3PO4) via an in situ process. The composite membranes were characterized using X-ray diffraction, FT-IR and TGA, which revealed the interaction between the sulfonic acid groups of SPPENK and BPO4. The SPPENK/BPO4 Composite membranes showed excellent thermal, oxidative and hydrolytic stabilities. The swelling ratios of composite membranes were about 5.0%, which was similar to that of SPPENK membrane. Proton conductivity of the composite membranes was studied under 100% relative humidity conditions up to 95 degrees C. A five-fold increase in proton conductivity was observed for the composite membrane with 30 wt.% BPO4. (C) 2008 Elsevier B.V. All rights reserved.