Membranes constituted from sulfonated poly (arylene ether sulfone) (SPES) and sulfonated poly(ether ether ketone) (SPEEK) are characterized using electron spin resonance (ESR) to monitor the local environment of the TEMPONE spin probe (2,2,6,6-tetramethyl-4-piperidone N-oxide). The probe occupies two phases of very different polarities and mobilities in both the swollen SPES and SPEEK membranes. ESR lineshape analysis allows the individual phases to be characterized in terms of their microviscosity, local polarity, and solvent ordering. The effects of sulfonation and methanol on the individual phase properties were studied by equilibrating different membranes with water-methanol mixtures over the full range of compositions. At sulfonation levels below 50%, SPES and SPEEK membranes have numerous but smaller, elongated hydrophilic domains in which lead to restricted motion and partial ordering of the probes in comparison with Nafion. The less polar phase of the aromatic polymers is attributed to a probe population associated with the polymer backbone. Methanol penetrates this region and disrupts the backbone structure at a concentration that depends upon the level of sulfonation. (C) 2010 Elsevier B.V. All rights reserved.