The molecular motions of sulfonated poly(ether-ether ketone) (SPEEK), synthesized by conventional method with a range of degree of sulfonation (DS) between 42 and 70%, as a function of DS and hydration were studied by H-1-C-13 dipolar recoupling by rotor-encoded longitudinal magnetization (RELM) and C-13 spin-lattice relaxations. The proton conductivity increased linearly with increasing DS from 52% and was comparable to that of Nafion 115 measured in the same condition for DS higher than 65%. The RELM dipolar patterns analyzed by the SIMPSON simulation program indicated that the population of phenyl rings in large amplitude motions such as 180 degrees-flips was reduced with increasing DS. On the other hand, T-1p (C-13) and T-1 (C-13) results suggested that the dynamic chains in both 66 kHz and 100 MHz regimes were more populated with increasing DS, possibly in small-amplitude oscillations. (C) 2009 Elsevier Ltd. All rights reserved.