The formation of PMMA nanoparticles of different size by continuous-flow emulsion polymerization was investigated by using a microreactor with an incorporated nozzle array. The emulsion was formed by pressing the organic phase containing the monomer MMA, a monomer-dissolved initiator as well as a cross-linking agent through an array of micro-pores into a fast streaming aqueous phase. High shear forces are realized by using low channel depths from 0.05 to 0.1 mm for the aqueous phase underneath the nozzle array. The array was prepared by a combination of an anisotropic crystallographic wet etching step and an anisotropic ion beam etching step in a Si (1 0 0) wafer. The nozzle array consists on a silicon frame and a window covered by a membrane of monocrystalline silicon with 16 x 48 micro-pores of equal diameter (20 mu m) in a rectangular arrangement. The liquids were actuated by a computer-controlled 4-channel syringe pump (Cetoni). Flow rates between 25 and 1000 mu L/min were applied. The formed emulsion was collected in a tempered reaction vessel (97 degrees C) for completion of the polymerization process. The obtained polymer nanoparticles were characterized by scanning electron microscopy (SEM) and by centrifugal sedimentation spectroscopy (DCS). Homogeneous particle populations with diameters between about 60 nm and 500 nm were realized in dependence on the composition of reactant mixture and process conditions. Larger particles (size between 260 and 500 nm diameter) were obtained in the absence of surfactants. Under optimized conditions, the process was robust against flow rate changes. The addition of a surfactant (sodium dodecyl sulfate, SDS) leads to a drastic reduction of the particle sizes. As result an easy way for the generation of PMMA nanoparticles in a micro-continuous-flow process and a convenient way to tune the particle sizes was developed. (C) 2010 Elsevier B.V. All rights reserved.