A graft copolymer, with hydrophilic poly(ethylene glycol) monomethyl ether (MPEG) side chains grafted onto a hydrophobic poly(dodecyl methacrylate) (PDMA) backbone, has been synthesized. The nonmodified backbone is soluble in cyclohexane. By contrast, the graft copolymer is soluble neither in cyclohexane nor in water but is well soluble in water/AOT/cyclohexane oil-continuous microemulsions (AOT = sodium bis-(2-ethylhexyl) sulfosuccinate). Already at low concentrations (1-3 wt %), the copolymer gives rise to a marked (up to 100-fold) increase in the viscosity in the microemulsion/copolymer system. The relative viscosity of the system is dependent on the characteristics of the microemulsion and can vary by 1 order of magnitude depending on the volume ratios of AOT, water, and cyclohexane. We interpret the results in terms of polymer-droplet cross-linking, caused by the solubilization of the hydrophilic side chains into the water droplets of the microemulsion. There are thus close analogies between our system and the more well-studied aqueous mixtures of hydrophobically modified water soluble polymers with micelle-forming surfactants.