The solubility parameter of sulfonated poly(ether ether ketone) (SPEEK) was investigated. SPEEK exhibited two solubility parameters, 26.4 and 35.7 J(1/2) cm(-3/2), and was similar in that respect to Nafion, which has two cohesive energy densities. The theoretical solubility parameter of SPEEK, 26.1 J(1/2) cm(-3/2), was determined with van Krevelen's method and corresponded to the experimental value. The theoretical volume fraction of SPEEK in the solvent was determined with Flory's equation. The trend of the theoretical volume fraction of SPEEK fit quite well with the experimental results when the solubility parameter of the solvent was lower than 35 J(1/2) cm(-3/2). A significant deviation of the experimental volume fraction of SPEEK with the high solubility parameter resulted from the presence of sulfonic acid groups. SPEEK in the sodium form exhibited similar solvent absorption in comparison with SPEEK in the acid form. (c) 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3128-3134, 2006.