The effect of hydrophobic and hydrophilic co-solutes on the theological properties of wormlike micelles of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) or sodium bromide (NaBr) was investigated. Monomeric (ethanol, 2-propanol, benzene and benzylic alcohol) and polymeric species (poly(ethylene oxide), poly(vinyl alcohol) and poly(propylene oxide), respectively PEO, PVA and PPO) of varying molecular weight were studied in order to assess the effect of co-solute 'length' on the interactions with the wormlike micelles. Rheological properties were characterised by the plateau modulus G(0) and the relaxation time tau(R) obtained from fits to the Maxwell model, and by the zero-shear viscosity eta(0). The rheological properties were unaltered by the addition of all hydrophilic solutes (up to 20 mM). With hydrophobic co-solutes instead, both eta(0) and tau(R) decreased considerably, while Go was unaffected. The effects were particularly remarkable with PPO for concentrations as low as 5 mM (ca. 0.3 g L-1), and tau(R) was seen to follow an exponential decrease with polymer M-w. The effect of the aromatic solutes (benzene and benzyl alcohol) on the rheology was highly dependent on the counterions used to induce micellar growth (Sal(-) or Br-), revealing a different type of interaction. Surprisingly, small-angle neutron scattering and Cryo-TEM measurements showed that the drastic changes observed in the rheology were not correlated to any visible structural change. Therefore the strong decrease in viscosity and relaxation time are to be attributed to other mechanisms than micellar break-up or rod-to-sphere transition. Crown Copyright (C) 2010 Published by Elsevier Inc. All rights reserved.