Abstract In this paper, the proton exchange membrane prepared by covalent-ionically cross-linking water soluble sulfonated–sulfinated poly(oxa- p-phenylene-3,3-phthalido- p-phenylene-oxa- p-phenylene-oxy-phenylene) (SsPEEK-WC) is reported. Compared with covalent cross-linked PEEK-WC membrane, this covalent-ionically cross-linked PEEK-WC membrane exhibits extremely reduced water uptake and methanol permeability, but just slightly sacrificed proton conductivity. The proton conductivity of the covalent-ionically cross-linked PEEK-WC membrane reaches to 2.1 × 10 −2 S cm −1 at room temperature and 4.1 × 10 −2 S cm −1 at 80 °C. The methanol permeability is 1.3 × 10 −7 cm 2 s −1, 10 times lower than that of Nafion ® 117 membrane. The results suggest that the covalent-ionically cross-linked PEEK-WC membrane is a promising candidate for direct methanol fuel cell because of low methanol permeability and adequate proton conductivity.