Solvent viscosity was found to be a predominant parameter to control free-radical surface graft polymerization. Poly(ethylene-alt-tetrafluoro-ethylene) foils were lithographically exposed to 92 eV photons in order to create patterns of radicals at their surface, which react with the oxygen of the ambient air to produce peroxides. Glycidyl methacrylate was then radically polymerized from these peroxide initiators. We discovered that increasing the viscosity of the grafting solution by addition of polyethylene glycol boosts the grafting reaction, leading to an order of magnitude increase in the height of the grafted layer. Further increasing the viscosity leads to thinner layers of grafted material. The increase of the grafted layer thickness in the low viscosity range is interpreted to be due to a diminution of the termination rate while for higher viscosity, the reduction of the initiation rate causes the observed decrease of the grafted thickness. This interpretation is in agreement with observed morphological and structural differences of nanostructures grafted at different viscosities. We conclude that viscosity is an essential and very easily controllable parameter for surface graft-reactions. (c) 2007 Elsevier Ltd. All rights reserved.