Porous monolithic polymers have been prepared by photoinitiated polymerization of mixtures consisting of 2-hydroxyethyl methacrylate, ethylene dimethacrylate, UV-sentitive free radical initiator and porogenic solvent within channels of specifically designed microfluidic chips and used as micromixers. Substituting azobisisobutyronitrile with 2,2-dimethoxy-2-phenylacetophenone considerably accelerated the kinetics of the polymerization. Mixtures of cyclohexanol and 1-dodecanol and of hexane and methanol were used, respectively, to control the porous properties and therefore the mixing efficiency of the device. The performance of the monolithic mixers has been tested by pumping aqueous solutions of two fluorescent dyes at various flow rates and monitoring the point at which the boundary of both streams completely disappears. Best results were achieved with a monolithic mixer containing very large irregular pores.