The effect of different ionic cosolutes (NaCl, Na(2)SO(4), Li(2)SO(4), 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 (ΔH(agg)(int)) and the standard free energy change of micelle adsorption on the macromolecule chain (ΔΔG(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, Na(2)SO(4), Li(2)SO(4), and NaSCN at 10.0 and 100 mmol L(-1), the cosolute presence lowered cac, 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.