Efficient mixing is an important component of micro total analytical systems that rely on chemical reactions such as DNA sequencing, cell lysis and protein analysis. However, laminar flow in these devices causes mixing to be diffusion-limited and slow. In order to increase the extent of reaction for on-chip fluorescent labeling of proteins a passive mixer was prepared by using UV light to photopattern a periodic arrangement of nanoporous polymer monolith (nPPM) structures directly within the channel of a plastic microfluidic chip. By optimizing the composition of the polymerization solution and irradiation time we demonstrate the ability to pattern nPPM in regular 100 μm segments. The mixing efficiency of this patterned array was evaluated by monitoring the laser-induced fluorescence intensity of the reaction product of fluorescamine and lysine introduced into a tee-junction