There has been increasing interest in “process intensification”, which means conducting processes in smaller volumes at higher flow rates and with increased yields. Rapid precipitation of both organic and inorganic compounds at high supersaturation requires homogenous mixing to control the particle size distribution. This is the basis of our process for making polymer protected nanoparticles via Flash Nanoprecipiation. We present the design and characterization of new micro-fluid-mechanical mixers based on fluid jets to drive turbulent mixing. Two and four jet mixers allow control of both the supersaturation and the final solvent quality by varying stream velocities. The designs also enable the separation of reactive components prior to mixing. We characterize the mixing performance of the two and four jet mixers using competitive fast reactions (the so called “Bourne reactions”). Adequate micromixing is obtained with a suitably defined Reynolds number when Re > 1600. The experimental results are compared to CFD simulations of the fluid mechanics and parallel reactions in the MIVM. Excellent correspondence is found between the simulation and the experimental results with no adjustable parameters. The CFD simulations provide a powerful tool for the optimization of these complex mixing geometries and for scaleup from laboratory to industrial scales. Further insights into the mechanisms of mixing are obtained by micro particle velocimetry measurements.