Poly(1,3-phenylene-5-phosphonic acid) is characterized by a high density of functional groups (less than 100 g/mol equivalent weight per proton), good thermo-oxidative stability up to at least 200 degrees C, the absence of a softening point, and a proton conductivity of more than 2 x 10(-3) S/cm between 110 degrees C and 160 degrees C under water vapor. Based on these properties, this polymer appears to be well suited as proton conducting component in polymer electrolyte membranes.