We systematically investigated the effect of incorporating a sterically hindered pyridyl group into a sulfo-phenylated polyphenylene to control the polymer's physicochemical properties through acid-base interactions. Homopolymers with similar molecular weights and comparable structures that vary by only one atom (N- vs C-) per repeat unit along the polymer chain were prepared. Compared to a non-pyridyl reference membrane, incorporation of a pyridyl group improves the oxidative stability against free radicals, increases the elongation at break to 55% (from 37%), and enhances the thermal stability to 326 degrees C (from 246 degrees C). In an accelerated fuel cell degradation test, polymeric membranes containing the sterically encumbered pyridyl unit exhibited exceptional stability (0.16 mV h(-1) degradation rate over 1000 h) and retained similar to 80% of their peak power density over this time.