A strong thixotropic viscosity behavior was observed when polyrotaxane prepared from a-cyclodextrins (CDs) and poly(ethylene glycol) (PEG) with a molecular weight of: 2000 was dissolved in dimethylsulfoxide (DMSO). A 10 wt% solution liquefied by vigorous shaking was rapidly gelated by standing - this sol-gel transition was reversible. The time for recovering the viscosity was dependent on the polyrotaxane concentration, i.e., a 10 Wt% Solution re,gelated within 30 s, whereas several hours were required for the gelation of a 2.5 wt% solution. The thixotropic nature of the Solution was also confirmed by the clockwise hysteresis Curve of the viscosity when the shear rate was increased and decreased. The gel permeation chromatography (GPC) measurement of the polyrotaxane in DMSO exhibited peaks in the high molecular weight region. The peak disappeared after the phenylcarbamoylation of polyrotaxane, suggesting that the peak was due to loose aggregations of polyrotaxane in DMSO. On the other hand, the DMSO solution of polyrotaxane prepared from CD and PEG with a molecular weight of 3350 whose inclusion ratio (51%) is slightly lower than that of PEG2000 polyrotaxane (72%) - neither demonstrated the abovementioned thixotropic viscosity nor the peak corresponding to the aggregations Occurring during the GPC measurement. The thixotropic behavior was speculated to be caused by the combined contribution of intermolecular attractive hydrogen bonding and higher rigidity of polyrotaxane prepared from PEG2000 than that of polyrotaxane prepared from PEG3350, presumably due to the higher inclusion ratio of the former than that of the latter. (C) 2007 Elsevier Ltd. All rights reserved.