The self-diffusion of water molecules, H+ and phenol-2,4-disulfonic acid molecules in phenol-2,4-disulfonic acid-polyvinyl alcohol membranes was investigated. The acid molecules are formed in clusters, therefore, the acid self-diffusion is restricted. The restriction size, which is about 10(-7) m, and degree of cluster connection are controlled by the membrane compound. The behaviour of protonic conductivity is very similar to the water molecule self-diffusion. The conductivity mechanism is vehicle type at high water content and relay-race type at low water content. (c) 2008 Published by Elsevier B.V.