We present the design, fabrication process, and the first test results of a high aspect ratio micromixer combined with a free jet for under 100 µs time resolved studies of chemical reactions. The whole system has been optimized for synchrotron small angle X-ray scattering (SAXS) experiments. These studies are of particular interest to understand the early stages of chemical reactions, such as the kinetics of nanoparticle formation. The mixer is based on hydrodynamic focusing and works in the laminar regime. The use of a free jet overcomes the fouling of the channels and simultaneously circumvents background scattering from the walls. The geometrical parameters of the device have been optimized using finite element simulations, resulting in smallest features with radius <1 µm, and a channel depth of 60 µm, thus leading to an aspect ratio >60. To achieve the desired dimensions deep X-ray lithography (DXRL) has been employed. The device has been tested. First the focusing effect has been visualized using fluorescein. Then the evolution and stability of the jet, which exits the mixer nozzle at 13 m s−1, have been characterized. Finally SAXS measurements have been conducted of the formation of calcium carbonate from calcium chloride and sodium carbonate. The fastest measurement is 75 µs after the beginning of the mixing of the reagents. The nanostructural evolution of chemical reactions is clearly discernible.