We apply methods of dynamic light scattering (DLS) and fluid mechanics to quantitatively establish the role of aggregation in the turbulent drag reduction of high molar mass poly(ethylene oxide) (PEO) solutions. By means of DLS, we show that the dilute aqueous solutions of high molar mass PEO (M-w similar to 4 x 10(6) g/mol) are aggregated and that this aggregate structure can be manipulated by addition of the chaotropic salt guanidine sulfate (GuS) or the divalent salt magnesium sulfate (MgSO4). In aqueous solution, we find Gamma similar to q(2.8 +/- 0.1), where Gamma is the DLS correlation function relaxation rate and q is the scattering vector. This scaling is consistent with internal motions of a large coil or aggregate. Addition of salt progressively decreases the scaling to Gamma similar to q(2.0 +/- 0.1) (at 0.5 M of MgSO4) consistent with center-of-mass diffusion of isolated coils. We further find that manipulating the aggregation state of PEO with MgSO4 shifts the critical condition for onset of turbulent drag reduction at dilute concentrations in pipe flow by a factor of 2.5. Because this critical condition is inversely proportional to the viscoelastic relaxation time of the polymer solution, we conclude that the aggregation state and the turbulent drag reduction behavior of PEO are strongly correlated. This correlation definitively confirms prior speculation (Cox et al. Nature 1974;249; Vlachogiannis et al. Physics of Fluids 2003:15(12)) that the high molar mass PEO commonly used in literature studies of turbulent drag reduction is in a state of aggregation. Furthermore, the quantitative differences in quiescent DLS characterization and turbulent flow pressure drop measurements suggest that high molar mass PEO undergoes flow-induced de-aggregation in transport systems with shear stresses as low as 0.5 Pa. (C) 2008 Elsevier Ltd. All rights reserved.