The interaction of lithium dodecyl sulfate (LiDS), sodium dodecyl sulfate (SDS), and cesium dodecyl sulfate (CsDS) with the water-soluble polymer poly(ethylene oxide) (PEO) has been studied in aqueous solutions by the electron spin resonance (ESR) technique using 2,2,6,6-tetramethylpiperidine-N-oxide (TEMPO) as a spin probe. The rotational correlation time of TEMPO has been obtained and there are substantial differences in tau(c) among LiDS, SDS, and CsDS with and without PEO. The results show that the microenvironment of the spin probe in the polymer-surfactant aqueous solutions depends strongly on the hydrated radius of the counterion. The headgroups of LiDS and SDS adsorbed on PEO are more tightly packed than those of the unperturbed micelles, which leads to a much more compact structure of the polymer-micelle complex. The headgroup packing of CsDS is less influenced by PEO.