The interaction of lithium dodecyl sulfate (LIDS) and sodium dodecyl sulfate (SDS) with poly(ethylene oxide) (PEO) has been studied with 16-doxyl stearic acid (16-DS) in aqueous solution by electron paramagnetic resonance (EPR). The rotational correlation times, and nitrogen hyperfine coupling constants A(N) versus the surfactant concentration with and without PEO have been calculated for different concentrations of 16-DS by our single-domain simulation program and EPRSIM program. Since the results of single-domain simulations were insufficient in pronounced changing the interval of r and A(N) values, spectra were also simulated with two and three domains to get better insight about domains. Better-fit results were found in these simulations. Data obtained from simulations revealed the effects of the polymer and surfactant counterion on the micelle formation and aggregation percentages. At about 8 mM SDS and about 12 mM LiDS concentrations, the presence of polymer provided more hydrophobic environment for 16-DS and this indicated that the location of the polymer was within the interface region. With the increase of the surfactant concentration, less hydrophobic environment was obtained for the nitroxide, which suggested a more open structure for the polymer-surfactant micelles.