Interactions between poly(propylene) glycols and sodium dodecylsulfate, in aqueous solutions were investigated through thermodynamic analysis. Densities and heat capacities, measured with the corresponding well-known Picker apparatus, were determined as a function of the surfactant concentration at 298.15 K. The apparent and transfer molar volumes and heat capacities of the solutes, surfactant and polymer, were deduced therefrom. From the analysis of their variations as a function of the surfactant concentration, it was shown that mainly strong hydrophobic interactions are involved between solutes, similar to the medium chain (C4–C6) 1-alcohols in equivalent surfactant systems. However, the large variations of these properties observed close to the critical micellar concentration signify profound structural changes in micelles caused by the presence of the polymer. Comparison with hexadecyltrimethyl ammonium bromide solutions shows a similar pattern of aggregation and complex formation although the strength and nature of the interactions are somewhat different.