Full molecular dynamics was used to simulate separately the diffusion of naked protons and H3O+ hydrated protons across sulfonated poly( phenyl sulfone) s. Simulations were carried out for wet membranes with the following characteristics: ion-exchange capacity, 1.8 mequiv/g of dry membrane; water uptake, 10-30%; temperature range, 300-360 K. The diffusion coefficient of naked protons is nearly 1 order of magnitude higher than that of the hydrated protons for the membranes with the lower water uptake (10%). For the membranes with higher water uptake the ratio between the diffusion coefficients of the two particles reduces to about half an order of magnitude. The conductivity of the naked protons increases from 21.4 x 10(-3) to 52.5 x 10(-3) S/cm when the water uptake increases from 10% to 30%. For hydrated protons the conductivity increases from 1.54 x 10(-3) to 7.57 x 10(-3) S/cm. The conductivities obtained through simulations carried out at 300 K for the hydrated proton across membranes with water uptake 18% and 30% are roughly similar to those experimentally measured for a membrane with ion exchange capacity = 1.8 mequiv/g and water uptake = 24.3%. Simulated conductivities of both naked protons and hydrated protons follow Arrhenius behavior.