A new class of cross-linkable poly(arylene ether ketone)s (cPAEKs) with sulfonic acid groups on both the backbone and pedant positions were synthesized. The chemical structures of the synthesized polymers were confirmed by 1H-nuclear magnetic resonance (NMR) and Fourier transform infrared (FT-IR) spectroscopy. The thermal and mechanical properties were investigated by TGA analysis and stress-strain testing. The major advantage of synthesized membranes is their exceptionally high proton conductivities. All the csPAEK membranes exhibited higher proton conductivity than Nafion® 117 over the temperature range from 40 - 90oC. In most cases, the cross-linked sPAEK membranes with 15% cross-linking density exhibited low methanol permeability, good thermal stability, and showed very high strain at break. The effects of sulfonation degree (SD) on proton conductivity, methanol permeability, water uptake, thermal and tensile properties of csPAEK membranes were investigated.