The rheological behavior of hydrophobically modified hydroxyehtyl cellulose (HMHEC) was studied in the presence of phospholipid vesicles. The effects of sonication, lipid species and concentration on the rheology were investigated at two HMHEC concentrations: 0.3 wt.% and 0.7 wt.%. It is found that compared to a pure HMHEC solution, the mixture viscosity could be enhanced by more than one order of magnitude for sonicated samples at certain lipid concentrations. The viscosity, which depends on the vesicle size and concentration, first increases with the lipid concentration and then decreases. Besides, the lipid addition increases the plateau modulus. These observed rheological behaviors can be explained by two types of association; some of the hydrophobes aggregate to form micelles, while others are embedded in the vesicle bilayers. By this means, the vesicles can be interconnected, contributing to the viscosity enhancement. (C) 2010 Elsevier Inc. All rights reserved.