An experimental investigation into the micromixing performance of coaxial mixers in a viscous system is reported, in which the iodide-iodate reaction system is chosen to quantitatively characterize the product distributions. The effects of feeding time, feeding position, impeller speed, inner impeller configuration, and operation mode on the segregation index, Xs, are examined. It is revealed that the feeding near the inner impeller benefits micromixing and should be regarded as the preferred position. The presence of the rotating outer impeller causes the micromixing performance of the coaxial mixer to be significantly better than the single-shaft mixer. While an increase in the outer impeller speed has a limited influence on micromixing, the inner impeller speed is the dominant influencing factor, that is, the Xs decreases obviously when the inner impeller speed is increased. On the other hand, the coaxial mixers with multiple and axial inner impellers have a better micromixing performance at the same specific power consumption, P-V, than that with single and radial inner impellers. Among the configurations consisting of a Rushton impeller (RT), six-straight-blade turbine impeller (SBT), and six-pitched (45 degrees)-blade turbine impeller (PBT), the Xs of the coaxial mixer is always the smallest at the same P-V when the PBT + RT configuration is used as the inner impeller. In addition, it is found that the difference in Xs that results from various operation modes is small in terms of power consumption; however, the co-rotation mode is still recommended for the micromixing of the coaxial mixer due to its excellent performance in general.