A parallel-competing chemical test reaction was performed in a rotating packed bed (RPB) with different radii to evaluate the micromixing efficiency. Experimental results indicated that increasing the rotational speed and the inner radius of the packed bed could intensify micromixing effectively. Increasing the outer radius of the bed had no significant effect on micromixing. In addition, experiments on the synthesis of magnesium hydroxide particles in an RPB and in a spinning disk reactor (SDR) were performed. Although higher micromixing efficiency was achieved and smaller particles were produced at a low liquid flow rate using an SDR, an RPB was a more effective device when the flow rate increased. Comparing with the reported segregation index of other mixing devices, a Higee system is a promising alternative as far as micromixing efficiency is concerned.