Microphase separation within hydrated Nafion (R) membranes was simulated using Dissipative Particle Dynamics (DPD). Morphologies were obtained at branching densities corresponding with equivalent weights ranging from similar to 800 to similar to 1400 (g/mole SO3) and water percentage volume contents ([H2O]) varying between 10% and 30%. All cases showed pronounced microphase separation involving a hydrophobic Teflon phase and a hydrophilic phase in which water is associated with SO3 groups that are located near the phase boundaries. Pore morphologies were found to depend strongly on water content and branching density. The average pore radius (R-pore) and the distance between the pores (Dcl-cl) were found to increase with water content obeying the relations R-pore = 1.3 + alpha[H2O] (nm), and Dcl-cl = 3.2 + beta[H2O] (nm). The values of the expansion coefficients alpha and beta decrease linearly with branching density with alpha = 5.3 x 10(-5) x (EW-450) and beta = 1.3 x 10(-4) x (EW-450) nm/vol%. For decreasing branching density the pores obtain a more spherical character. The consequence of this on water diffusion is estimated by employing Monte Carlo trajectory calculations in which we assume that water movement is confined within the hydrophilic phase and local water mobility to be equal to that of pure water. The estimated diffusion constants increase linearly with branching density (i.e. linear decrease with equivalent weight). Experimental water diffusion constants obtained from literature for Nafion] 100 membrane are in good agreement with our calculations. A counterintuitive picture evolves in which smaller pores lead to enhanced water diffusion. (C) 2009 Elsevier B.V. All rights reserved.