The thickening, properties of aqueous Solutions of HHM-HEC (hydrophobically-hydrophilically modified hydroxyethylcellulose) and the emulsification mechanisms of HHM-HEC/water/oil systems were investigated. A dramatic increase in viscosity was observed with increased HHM-HEC concentration in water. caused by aggegation of hydrophobic alkyl chains. At higher concentrations of HHM-HEC (above 0.6 wt%) in water, it forms an elastic gel, which has good thixotropic properties and a high yield value. O/W (oil-in-water) type emulsions were obtained using HHM-HEC, which can emulsify various kinds of oil, including hydrocarbon. silicone, and perfluoropolymethylisopropyl ether. The viscosity of these emulsions depends Only upon the oil volume fraction, not oil the kind of oil. In addition. the oil particle size in the emulsions remained constant after a certain period because HHM-HEC formed a strong gel network Structure and a protective layer, which prevented the emulsion front coalescing. Measurements of interfacial tension revealed that the alkyl chains in HHM-HEC did not significantly lower the interfacial tension at the water/oil interface when 0.5 wt% of HHM-HEC was added to water. Steady flow and oscillatory experimental results Show that the rheological behavior of HHM-HEC/water-/oil emulsions was similar to that of aqueous solutions of HHM-HEC. In the HHM-HEC/water/Oil emulsion system, oil droplets were dispersed and kept stable in the Strong let Structure of HHM-HEC. The aqueous Solution of HHM-HEC showed salt resistance. It is thought to be due to sulfonic acid groups in HHM-HEC. The stability of the emulsion Using HHM-HEC is based on both protective colloidal effects and associative thickening caused by alkyl chains in HHM-HEC. (C) 2004 Elsevier Inc. All rights reserved.