Mixtures of polymers and surfactants are widely used in aqueous media to provide structure, solubilization domains, and/or colloidal stability [Tsianou & Alexandridis: Surfactant Sci. Ser. 2005, 124, 657]. Underlying these functions are inter- and intra-molecular interactions that depend on (i) the nature of the polymers, (ii) the type of surfactants, (iii) solution conditions (e.g., salinity, pH), and (iv) external stimuli (e.g., temperature, shear). The solution behavior and interactions of associating polymers with surfactants can be profoundly affected by the presence in water of polar organic solvents (cosolvents) which are ubiquitous in pharmaceutical, coating and home/personal care formulations. This presentation will discuss cosolvent effects in terms of changes in the polymer conformation and the surfactant self-assembly obtained from small-angle neutron scattering (SANS) measurements and complementary techniques. Examples will be presented from poly(ethylene oxide) or cellulose ether solutions in the presence of the sodium dodecyl sulfate surfactant (hydrogenous or deuterated, to enable contrast variation SANS).