Series of sulfonated poly(ether ether ketone)s (SPEEKs) were prepared by sulfonation of commercial Victrex((R)) and Gatone((R)) PEEK for a comparative study of proton exchange membranes (PEM) intended for fuel cell applications. The degree of sulfortation (DS) of the sulfonated PEEK was determined from deuterated dimethyl sulfoxide (DMSO-d(6)) solution of the purified polymers using H-1 NMR methods. The second method using a solvent suppression technique, in which DS results were obtained directly from H-1 NMR spectra of SPEEK dissolved in sulfuric acid (non-deuterated) reaction medium was evaluated. The variation between the two methods was determined. The room temperature sulfonation of PEEK, monitored directly by second H-1 NMR method, proceeded rapidly initially, reaching DS similar to 0.8 within I week, but progressed slowly thereafter. A maximum DS of 1.0 was determined after I month at ambient temperature (similar to22degreesC). The thermal properties of SPEEK were characterized by means of DSC and TGA. The mass averaged molecular weights M-w of both Victrex((R)) and Gatone((R)) PEEK were estimated from intrinsic viscosities measured in sulfuric acid solutions. It was verified that higher temperature (55 degreesC) did not induce any apparent chain degradation of Victrex((R)) (or Gatone((R))) PEEK by M-w tests. The water uptake and swelling properties of prepared films were studied and the proton conductivities at different temperatures were measured. The conductivities of the SPEEKs were found to increase with increasing DS and temperatures. The effect of film casting solvents on the conductivities is also discussed. (C) 2003 Elsevier B.V. All rights reserved.