The ionic conductivity of the ammonium salt of 12-tungstophosphoric acid has been determined in dry Ar, at 100% RH and in presence of liquid water using impedance techniques. The conductivity increases from sigma = 10(-5)-10(-4) S cm(-1) for the material, stored in dry Ar to sigma = 1.1-1.9 x 10(-2) S cm(-1) at 100% RH. Mixed with 10% water the material gains a very high electrical conductivity in the range 3.3-8.0 x 10(-2) S cm(-1) when the temperature varies from ambient to 60 degrees C. The dominant mobile charge carrier in this system seems to be hydrated H+. Water plays a decisive role in conductance, providing both charge carriers and a vehicle for their motion.