The absorption of vanadium to Nafion® was investigated through ex-situ isotherm and conductivity measurements at 23 °C. The data show a maximum loss of ion exchange capacity (IEC) of 30% for all four oxidation states of vanadium. The affinity of vanadium for N115 was measured by back titration and atomic absorption (AA) and characterized by isotherms at 23 °C, and the affinity is highest for the divalent species and lowest for the pentavalent species in the following order: VO2 + (V5+) < VO2+ (V4+) < V3+ < V2+. Steric hindrance from the associated water complex may explain the lower absorption of vanadium compared to alkali metals. The conductivity for the VO2+ (minimum affinity)-exchanged membrane was 2–3× lower than the sodium-exchanged membrane at an approximate RH = 100%.