in an aqueous solution of a hydrophobically, end-modified poly(ethylene glycol) polymer, HM-PEG, the thickening effect is dependent on intermolecular hydrophobic associations and the formation of a network structure. In the present investigation cyclodextrin, CD, has been added to an aqueous HM-PEG solution and a decrease in Newtonian viscosity has been followed. The decreased viscosity refers to polymer-polymer associations becoming less numerous when complexes between polymer hydrophobic tails and CD become more frequent; CD-decorated polymer hydrophobic tails have no possibility of contributing to the network. It was found that deactivation of the first few hydrophobic tails has very large consequences for the viscosity. A termination of a fraction as small as 10% (or below) of the total amount of polymer hydrophobic tails may reduce viscosity to a level almost corresponding to that of the unmodified parent polymer. This can be understood by taking into account that a solution of a HM-PEG polymer is, expected to be inhomogeneous with large concentration fluctuations and that the viscosity is likely to be strongly decreased by reducing the probability of hydrophobic associations responsible for connecting different blusters. The effect of CD on rheology is very different for different architectures of hydrophobically modified polymer, in particular between graft copolymers and end-capped ones. This can be understood from the differences in network structure.