The effect of different ionic cosolutes (NaCl, Na2SO4, Li2SO4, NaSCN, Na-2[Fe(CN)(5)NO], and Na-3[Co(NO)(6)]) on the interaction between sodium dodecyl sulfate (SDS) and poly(ethylene oxide) (PEO) was examined by small-angle X-ray scattering (SAXS) and isothermal titration calorimetric techniques. The critical aggregation concentration values (cac), the saturation concentration (C-2), the integral enthalpy change for aggregate formation (Delta H-agg(int)) and the standard free energy change of micelle adsorption on the macromolecule chain (Delta Delta G(agg)) were derived from the calorimetric titration curves. In the presence of 1.00 mmol L-1 cosolute, no changes in the parameters were observed when compared with those obtained for SDS-PEO interactions in pure water. For NaCl, Na2SO4, Li2SO4, and NaSCN at 10.0 and 100 mmol L-1, the cosolute presence lowered cue, increased C-2, and the PEO-SDS aggregate became more stable. In the presence of Na-2[Fe(CN)(5)NO], the calorimetric titration curves changed drastically, showing a possible reduction in the PEO-SDS degree of interaction, possibility disrupting the formed nanostructure; however, the SAXS data confirmed, independent of the small energy observed, the presence of aggregates adsorbed on the polymer chain.