Fluorescence probe techniques together with microcalorimetry and dye solubilization were used to study the interaction between nonionic polymers and anionic surfactants with different monovalent counterions in order to examine the effects of the counterion. The polymers used were the cellulose ethers hydroxypropyl methyl cellulose (HPMC) and ethyl hydroxyethyl cellulose (EHEC). The surfactants were dodecyl sulfates with potassium, sodium, and lithium as counterions (KDS, NaDS, LiDS). The counterion influenced the interaction start concentration as well as the nature of the mixed aggregates formed. The interaction start, according to surfactant concentration, was found to be in the order KDS < NaDS < LiDS for both polymers as well as in aqueous solution. From fluorescence measurements it was found that the KDS-polymer aggregates shield pyrene from water better than the other surfactants, indicating larger aggregates with a more fluid interior. The microcalorimetry measurements confirm that the adsorption of the surfactants onto the polymer is endothermic and entropy driven at the start and as more clusters are formed on the polymer chains the process converts to being exothermic and driven by both enthalpy and entropy.