Nanoparticles can be synthesized by precipitation. State-of-the-art is the precipitation of nanoparticles in stirred tanks or high-pressure T-mixers. The study presents two new reactor concepts for continuous precipitation of nanoparticles. Both of them utilize ultrasonic sound as a mixing accelerator. The first reactor has a conical chamber (10 mL), which is used to study the micromixing quality, the cavitation intensity and the precipitation of barium sulfate nanoparticles. The second reactor has a so-called cavitational chamber (2.5 mL), which is an optimized conical reactor. Both reactors are compared with each other with respect to the properties of the products. Additionally, the influence of the ultrasonic output from the transducer to the liquid and the feed rate are demonstrated. (C) 2011 Elsevier Ltd. All rights reserved.