Earlier experiments at small scale (0.037 m(3); nozzle diameter 0.008 m) showed locally high levels of turbulent energy dissipation (10(3)-10(4) W kg(-1)) in jets when nozzle velocities were 4-12 m s(-1). Such levels promote the dispersion of immiscible fluids and the control of the product distributions of mixing-controled, multiple chemical reactions. The present work studies the scale-up of jet reactors. Product distributions of fast reactions were measured at small scale in turbulent viscous and aqueous solutions as well as using two larger nozzles (0.012 m and 0.031 m) and two larger semi-batch reactors (0.10 m(3) and 0.25 m(3)). The influence of mesomixing at these two scales could not be eliminated and no scale-up criterion for micromixing could be established. A new mesomixing time constant, calculated from the feed flow rate and the nozzle diameter and velocity, was derived from Corrsin’s time constant for inertial-convective mass transfer. It gave an approximate, but useful and easily applied correlation of 50 product distributions measured at the three jet reactor scales. It also agreed well with experimental results for semi-batch stirred tank reactors when mesomixing controlled the product distribution.