The microviscosity of hydrophobically modified hydroxyethyl cellulose (HMHEC) aqueous solutions is experimentally determined by conductometry with added ions as probe. Compared to its bulk viscosity, the microviscosity of HMHEC solution could be lower by four orders of magnitude. Since the electric conductivity reduction of added NaCl is almost the same for HMHEC and its unmodified counterpart at an equal weight concentration, one can conclude that the hydrophobic modification for the polymer hardly has any effect on the solution's microviscosity. (C) 2008 Elsevier Inc. All rights reserved.