Four surfactants, namely, sodium n-decyl sulfate (SDeS), sodium n-hexadecyl sulfate (SHS), sodium n-dodecyl sulfate (SDS), and Triton X-100, were used as additives to Study thermal behavior and sol-gel transformations in dilute aqueous hydroxypropyl methyl cellulose (HPMC)/surfactant mixtures using micro-differential scanning calorimetry. The influence of anionic surfactant, SDS on the gelation varied with SDS concentration where the sol-gel transition started at a higher temperature. Shape of the thermograms changed from single mode to dual mode at the SDS concentration of 6 mM and higher. SDeS and SHS, however, resulted in "salt-in" effect of a different magnitude during gelation. Triton X-100, being a non-ionic surfactant, showed a minor "salt-out" effect on the thermo-gelation process. On the basis of different thermal behavior of anionic and non-ionic surfactant/HPMC systems, a mechanism is proposed explaining how the chemical structure and electro-charge of the surfactants affect the polymer/surfactant binding and polymer/polymer aggregation because of hydrophobic interaction during the sol-gel transition. (C) 2009 Wiley Periodicals, Inc. Journal of Applied Polymer Science 113: 2887-2893, 2009