The ethylene oxide (EO) mobility in polystyrene-gratr-[poly(ethylene oxide)] (PS-g-PEO) and polystyrene-graft-[stearyl poly(ethylene oxide)] (PS-g-SPEO) copolymers was evaluated by spin probe techniques. The ESR spectra indic ate that 4-hydroxyl-TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl) is strongly biased to the PEO phase of the PS-g-(S)PEO membranes. The rotational correlation time tau(c) can also be employed to assess the PEO mobility in PS-g-(S)PEO membranes. Although tau(c) of PS-g-(S)PEO usually decreases with increasing surface density of EO, it is of interest that tau(c) is rather high when the surface within a depth of at least 5 nm is fully occupied by SPEC (sample PS-g-SPEO-72.6).