In the present paper we show that polystyrene–based copolymers, which carry a defined amount of photo–reactive benzophenone moities can be transferred and immobilized to paper substrates via a simple dip coating approach and subsequent illumination of the paper substrates with UV-light. Non-bound macromolecules can be removed from the cellulose fibers by solvent extraction. Thereby, the amount of immobilized polymer can be adjusted over a wide range by changing the polymer concentration in the dip coating solution. The resulting polymer-modified paper substrates were characterized using IR spectroscopy, scanning electron microscopy (SEM), fluorescence microscopy and static contact angle measurements. The polymers are attached to cellulose fibers using a photo–chemical approach and stable chemical micro patterns, including paper-defined microchannels, can be designed inside model paper substrates by using conventional UV-lithography. These channels are capable to control the fluid penetration by capillary actions. An engineering of the paper substrate itself allows to modulate the speed of the fluid transport of an aqueous solution inside paper-defined microchannels. The latter will become important for a number of applications.