Polymer composite membranes based on sulfonated poly(phthalazinone ether sulfone) (SPPES) and cerium sulfophenyl phosphate (CeSPP) are prepared. Three CeSPP concentrations are used: 10, 20, and 30 wt.%. The membranes are characterised by infrared spectroscopy (IR), X-ray diffraction spectroscopy, thermal gravimetric analysis, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The IR results indicate the formation of intense hydrogen bonds between CeSPP and SPPES molecules. The SEM micrographs show that CeSPP well dispersed in composite membrane. The properties of the membranes are evaluated by their water uptake, ionic exchange capacity, proton conductivity and methanol permeability. The proton conductivity of the SPPES (DS 91%)/CeSPP (30 wt.%) composite membrane (I) reaches 0.384 S/cm at 130 degrees C and 100% relative humidity, which is three times more than Nafion (R) 117. CeSPP improves the conductivity of composite membranes at a low humidity. At 105 degrees C and 70% RH, the proton conductivity of membrane (I) is 9.1 x 10(-2) S/cm, while Nafion (R) 117 8.8 x 10(-3) S/cm. The methanol permeability of membrane (I) is 10(-8) cm(2)/s. That is much lower than Nafion (R) 117. (c) 2012 Elsevier Ltd. All rights reserved.