2 to 4% aqueous solutions of regioselectively substituted 3-mono-O-ethyl(3-EC) and randomly substituted ethyl- (EC) cellulose show association and gelation above 50-60 and 20-30 degrees C, respectively. In contrast, almost no association or gelation was seen with regioselectively substituted 3-mono-O-methoxyethyl cellulose (3-MEC). This could be assessed by eye and measured using the techniques of H-1 high-resolution nuclear magnetic resonance, oscillatory rheology, and micro-differential scanning calorimetry (DSC). Several processes are thought to occur during association and gelation of EC and 3-EC. These were separated with respect to temperature, to varying degrees, on cooling and have been modeled using DSC data. In accordance with other work in the literature, they are thought to be, on heating, the disassociation of structures in solution followed first by the formation of water cages and then by the association of hydrophobic moieties into a precipitate or gel. Contrasting with previous work (Haque and Morris, Carbohydr Polym 1993, 22, 161-173), the rheological measurements do not show "two distinct waves of structure formation" at the present concentrations and the DSC measurements show, on cooling, an extremely wide separation in temperature of the processes for the regioselectively substituted cellulose derivative 3-EC. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1743-1752, 2009