Sulfonated poly(ether ether ketone) (SPEEK)/boron phosphate (BPO4) composite membranes for polymer electrolyte fuel cells (PEFCs) were prepared via an in situ sol-gel process in our previous study [P. Krishnan, J.-S. Park, C.-S. Kim, Preparation of proton-conducting sulfonated poly(ether ether ketone)/boron phosphate composite membranes by an in situ sol-gel process, J. Membr. Sci. 279 (2006) 220]. Proton conductivity of the composite membranes increased with increasing the amount of BPO4 up to 40 wt.% due to the increasing BPO4 particle size. In this study, the effect of several variables such as reaction time, reaction temperature and ionic form of solution-casting SPEEK polymers on the relationship between the size of inorganic particles and proton conductivity of the membranes was investigated for the preparation of SPEEK/BPO4 composite membranes. The composite membranes were characterized using FT-IR, SEM, DSC, TGA and proton conductivity. Among the variables, reaction time and reaction temperature were not dependent on the proton conductivity and size of BPO4 in the in situ sol-gel process. However, the ionic form of solution-casting polymers for the composite membranes significantly influenced the size of BPO4 particles. At the same amount of BPO4 in the composite membranes, the composite membrane using W-form polymer made the BPO4 particle size five to six times higher than that using Na+-form, polymer. Water uptake of the former composite membrane (in particular, an amount of freezable water) was higher than the latter due to the bigger size of BPO4, and the proton conductivity of the former composite membrane increased. As a result, it can be recommended that the preparation of the composite membranes using H+-form polymer is preferable via the in situ sol-gel process. (c) 2008 Elsevier B.V. All rights reserved.