Sulfonated polysulfone (SPSU) ionomer membranes were prepared via a solution sulfonation method using mixtures of chlorosulfonic acid (HSO3Cl) and chlorotrimethylsilane ((CH3)(3)SiCl) as sulfonating agent. Ion exchange capacities (IECs) of SPSU ionomer membranes were controlled by varying the amount of HSO3Cl. In the present study, we investigated the thermal transition behavior of SPSU membranes annealed at different temperatures (60 and 150 degreesC) below glass transition temperature (T-g) of SPSU ionomer membranes. and also studied the annealing effect on the proton conductivity and methanol transport through these ionomer membranes. The changes in T-g (degreesC), ion exchange capacity (lEC. mmol/g), water uptake content (%). proton conductivity (S/cm), and methanol permeability (cm(2)/s) were reported and discussed. The heat-treatment of SPSU membranes at higher temperature led to the increase, in T-g and simultaneously the decrease in water uptake, proton conductivity and methanol permeability. Here, it was found that the annealing below T-g accellerated the equilibrium process and physical aging of SPSU ionomer membranes and Subsequently induced more compact chain packing structure, which eventually affected the proton and methanol transport through these ionomer membranes. (C) 2004 Elsevier B.V. All rights reserved.