Composite membranes of sulfonated poly(ethersulfone)/1,1-carbonyl diimidazole/1-(3-aminopropyl)-silane/silica (SPES/CDI/AS/SiO 2 ) with silica of various contents (3, 5 and 8 wt%) were prepared as electrolytes for direct methanol fuel cells (DMFCs). Comparison was made with pure SPES and SPES/SiO 2 . The properties of the composite membranes were studied by FTIR, TGA, XRD, water and methanol uptake, proton conductivity. SPES/CDI/AS/SiO 2 membranes were also characterized by scanning electron microscopy (SEM), which showed good adhesion between the modified sulfonic acid (-SO 3 H) groups of SPES and silica because of cross-linking with covalent bond formation and reduced cavities in the composites. This effect played an important role in reducing water uptake, methanol uptake and methanol permeability of the SPES/CDI/AS/SiO 2 composites. The water and methanol uptake and also methanol permeability of the SPES/CDI/AS/SiO 2 composite membrane with 8% SiO 2 were found in the order 3.58%, 2.48% and 1.91×10 −7 (cm 2 s −1 ), lower than those of SPES and Nafion 117. In SPES membrane of 16.94% level of sulfonation, the proton conductivity was 0.0135 s/cm at 25 o C, which approached that of Nafion 117 under the same conditions. Also, the proton conductivity of the SPES/CDI/AS/SiO 2 8% membrane was 0.0186 s/cm, which was higher than that of SPES at room temperature. The preparation of SPES/SiO 2 composites in the presence of AS and CDI, led to 63%, 56% and 64% reduction of water uptake, methanol uptake and methanol permeability, respectively without a sharp drop in proton conductivity of the composite membranes which featured a good balance between high proton conductivity, water and methanol uptake of SPES/CDI/AS/SiO 2 membranes.