Microfluidic synthesis allows for a good control of the particle formation conditions while minimizing the consumption of material. In this study, we exploited these advantages for the nonaqueous synthesis of TiO2, ZnO and CeO2 nanoparticles in a closed micro droplet reactor which resulted in well-defined particle structures. Monodisperse droplets are generated in amicrofluidic flow-focusing area and subsequently transported to a reaction zone allowing synthesis temperatures of up to 200 degrees C. In addition, we showed that a continuous particle generation process using solid precursor species can be realized without channel clogging. We present different microfluidic designs with reaction zones consisting either of a long meandering channel or a short expansion channel to realize the different reaction times required for the respective model systems. Interestingly, we found that at elevated temperatures, the reaction mixture diffuses into the surrounding continuous phase fluid which leads to a shrinkage of the droplets, forcing eventually the assembly of highly spherical aggregate structures. Depending on the different model systems, we obtained nanospheres of about 100 nm from the CeO2 synthesis and microspheres up to 17 mu m in size from the TiO2 synthesis. In contrast, ZnO syntheses at milder temperatures were conducted without any droplet shrinkage which resulted in randomly shaped aggregates of 150 nm that self-assembled in the droplet micro reactor. Thus, a versatile synthesis technique is presented that can be applied for various metal oxides over a broad range of reaction parameters, as a promising approach for the controlled production of high quality nanoparticles. (C) 2018 Elsevier Ltd. All rights reserved.