Pervaporation separation of ethylene glycol aqueous solutions was carried out using sulfonated poly(ether ether ketone) homogeneous membranes. Membrane relaxation in separation processes was observed and alleviated by heat treatment. The heat-treated pervaporation membranes experience further relaxation in the separation process by the swelling interactions of the feed mixtures before steady-state transport is reached. Membrane performance is investigated in terms of sorption and pervaporation separation. The preferential sorption and diffusion of water in the membranes were observed. Membrane performance can be interpreted by the modified solution-diffusion model, which takes into account the plasticization interaction of the transporting species in the membrane. Some parameters in this model were estimated by fitting the related experimental data. The benefit of using pervaporation for ethylene glycol dehydration is discussed in terms of energy consumption.