We present the effect of the degree of sulfonation (DS) and water content on the phase separation in sulfonated poly(ether ether ketone) (SPEEK) ionomers, using the method of spin probe electron spin resonance (ESR). The ionomer contained the nitroxide radical 4-oxo-TEMPO (TEMPONE) as the spin probe. ESR spectra in the temperature range 110-350 K were measured in order to obtain information on the dynamics of the spin probe, and the local polarity of the probe site in SPEEK membranes with DS = 38%, 58%, and 70%. ESR spectra at 110 K and at 350 K were simulated for each DS value, in samples soaked at ambient temperature and at 60 degrees C. For all samples the hyperfine splittings for the N-14 nucleus, aiso, was in the range 15.4-15.8 G, indicating a hydrophilic environment of the probe, based on the comparison with the aiso value measured for TEMPONE in bulk water at 350 K. The local dynamics of the probe is reflected in the type and number of spectral components as a function of DS and type of soak. At 350 K spectra consisting of one slow spectral component only were determined for DS = 38% and 58% after ambient soak, and a single fast component was detected for DS = 70% after the 60 degrees C soak; spectra consisting of two spectral components, fast and slow, were seen for DS = 38% and 58% after 60 degrees C soak, and for DS = 70% after ambient soak. The ESR spectra together with the simulations and the water content indicated a different structure and phase separation as a function of DS: For the lowest DS, 38%, the results suggest the presence of small hydrophilic pockets where the spin probe is located in bound water, and slightly larger water domains after the 60 degrees C soak. The appearance of the fast component in DS = 58% after the 60 degrees C soak is compatible with larger water domains, which can also accommodate a fast spectral component. In DS = 70% after the 60 degrees C soak, the dominant relative intensity of the fast spectral component suggests the existence of large and connected water channels. We conclude that the phase separation in SPEEK ionomers is a function of DS and water content.