In most chemical reactions, reaction rates increase with increasing reactant concentrations. In this study, we report an unusual catalytic oxidation reaction with a negative reaction order, in which the reactant concentration inversely affected the oxidation rate. In the reaction, trypan blue was oxidized by hydrogen peroxide with copper-triglycine as a catalyst. Under a strong alkaline condition, the reaction rate was inversely proportional to the hydrogen peroxide concentration (i.e., the reaction rate was faster when the hydrogen peroxide concentration was lower). Without the copper-triglycine catalyst, the phenomenon did not happen. A possible explanation was that hydrogen peroxide competed with the reactive species of the catalyst and slowed down the reaction. This phenomenon had an important implication in advanced oxidation processes for wastewater treatments. To achieve a faster oxidation rate of trypan blue, one should add hydrogen peroxide slowly to keep its concentration low at all times. On the basis of this principle, we developed a continuous microdroplet injection process to deliver the hydrogen peroxide solution as droplets. This process was faster and more efficient than a batch process for the degradation of trypan blue.