Poly(ether ketone ether sulfone) (PEKES) was synthesized by nucleophilic aromatic substitution polycondensation between bisphenol S and 4,4'-difluorobenzophenone (System A), and between bisphenol S and 4,4'-dichlorobenzophenone (System B). Properties of both post-sulfonated polymers are compared with a commercial PEEK 150XF from Victrex(A (R)). The sulfonated polymer samples were characterized by FTIR, H-1-NMR, TGA, and LCR meter, and the degree of sulfonation (DS) was determined. Advantages of the synthesized PEKES is the better solubility of PEKES in H2SO4 relative to PEEK 150XF which results in higher sulfonation degrees, and the S-PEKESs backbones contain the aromatic rings, the carbonyl groups (-C=O) and the sulfone groups (-SO2-), which promote the chain stiffness and thermal stability. The ion exchange capacity (IEC), the water uptake (%), the dielectric permittivity (epsilon'), and the electrical conductivity increase monotonically with increasing DS.