Combination of a neutralization reaction with the iodine-iodate reaction 5I^- + IO₃^- + 6H⁺ = 3I₂ + 3H₂O has been used for characterizing micromixing efficiency, since the former reaction is much faster than the latter reaction. But one of the problems of this technique is that it is not easy to prevent the iodine produced by the latter reaction from decomposing. We investigated three neutralization reactions in order to find one that allows stable analysis of a sampled solution without further decomposition of iodine. The neutralization reactions investigated were (1) H₂BO₃^- + H⁺ = H₃BO₃, (2) CH₃COO^- + H⁺ = CH₃COOH and (3) HPO₄^2- + H⁺ = H₂PO₄^-, in which the reaction (3) gave the most stable condition for analysis in a beaker test. We applied the third reaction system to characterizing micromixing efficiency in a rotating disk reactor under continuous operation. The segregation index, whose value between 0 for perfect mixing and 1 for total segregation, was measured by changing the feed rate and feed position on the rotating disk at the rotational speed of 600, 800 or 1000 rpm. The segregation index increased with the feed rate but decreased with the rotational speed. The maximum segregation index was less than 0.1 at 600 rpm and 0.05 at 100rpm. These values, which are much smaller that those measured in agitated vessels, indicate that good micromixing is attained in a thin film flow formed on the rotating disk reactor.