This work quantitatively measures the micromixing efficiency in a Couette flow reactor on the basis of a unique parallel competing reaction scheme proposed by Villermaux and co-workers. According to those results, hydrodynamic instability markedly affects the segregation index. In laminar Couette flow (LCF) regime, increasing the rotational speed only slightly increases the micromixing efficiency. On the other hand, initiating the Taylor vortex motion significantly enhances the micromixing efficiency. In addition, the mixing times are estimated on the basis of an incorporation model. In LCF regime, the estimated micromixing time is unreasonably high. At the subsequently hydrodynamic transitions, t(m) substantially decreases and ultimately reaches a low level of 10(-3)-10(-4) s in the turbulent vortex flow regime.