High-sensitivity differential scanning calorimetry (HSDSC) experiments and phase studies have been combined in order to investigate thermal events, possibly related to thermoreversible gelation, in semidulute aqueous systems of ethyl(hydroxyethyl)cellulose (EHEC) and sodium dodecyl sulfate (SDS) in the temperature rage 20-80 degrees C. The phase behavior of two gelling EHEC samples in 0.50 and 1.00 wt % aqueous solution containing varying amounts of SDS has been characterized. Both samples formed thermoreversible gels in 1.00% solutions at SDS concentrations above 2 mmol kg(-1), while in 0.50 wt % solution only one sample formed a gel nd now in a restricted region of temperature and SDS concentration. Large, well-defined endothermic peaks were seen in DSC traces of the gelling EHEC samples in purewater. The peaks appeared at about the clouding temperature (CP) of the solutions. Surfactant-free solution of a more hydrophilic (nongelling) EHEC only gave a small heat capacity shift, at CP. The transition peaks observed for the gelling EHEC in pure water are considered to indicate that polymer association through interaction between the hydrophobic segments precedes the liquid-liquid phase separation. The addition of SDS to solutions of the gelling EHEC significantly changed the features of the DSC peaks, which indicates that SDS changes the association behavior of the polymer as the clouding temperature is approached. At certain compositions the phase separation is arrested and instead a gel forms.