The first attempt to investigate polymer-surfactant interactions in gelling and nongelling aqueous mixtures of a nonionic cellulose ether and a surfactant by means of vibrational spectroscopy is presented. A series of aqueous solutions of ethyl(hydroxyethyl)cellulose (EHEC) with addition of anionic surfactant sodium dodecyl sulfate (SDS) of different concentrations was thoroughly investigated by means of Raman scattering and FTIR absorption techniques. Our data show that even in the sol state (i.e., below the gel point) there are interactions between the polymer and the surfactant, and both bound and free surfactant molecules were detected. This interaction, which cannot be characterized as a chemical one, occurs mainly between the side chains of the polymer and the sulfonic acid groups of SDS. Above the gel paint, a new type of interaction appears, which mainly involves the SO3- groups and water molecules. The intermolecular interactions are studied vs changes of both temperature and polymer-surfactant compositions, and a possible model for the gelation process is discussed.