The synthesis and conductivity of highly phosphonated polypentafluorostyrene are demonstrated. Efficient postphosphonation (90%) was achieved via the classical nucleophilic aromatic substitution (S(N)(Ar)) Michaelis-Arbuzov reaction of polypentafluorostyrene (PFS) with tris(trimethylsilyl) phosphite. The contrivance is in the cumulative electron-withdrawing effect of the fluorine functions. This simultaneously facilitates the S(N)(Ar) reaction and enhances acidity of the resulting phosphonic acid. The most important consequence is a substantial increase of the H(+) conductivity being the highest one measured on phosphonated polymer: sigma = 0.1 S cm(-1) at 108 degrees C, p = 10(5) Pa water vapor pressure. This value is 4 times higher than the one for poly(vinylphosphonic acid) (sigma = 0.025 S cm(-1)) and higher than Nafion 117 (sigma = 0.075 S cm(-1)) under the same conditions. Additionally, this polymer showed outstanding resistance to oxidative and thermal treatment (T(decomp) = 340 degrees C at 70% O(2) atmosphere). All this makes the phosphonated PFS a very promising candidate as polymer electrolyte for fuel cell applications.