The catalytic oxidation of 1,2-dihydroxy-benzene-3,6-disulfonate (tiron) by metal ions has been studied for detection of the metal ions in capillary electrophoresis (CE). Although Co2+ shows the strongest catalytic capability, some other metal ions also catalyze this reaction. If metal ions encounter a H2O2 zone after electrophoretic separation in the running buffer containing tiron, tiron is catalytically oxidized while the metal ion passes through the H2O2 zone. Anionic tiron radicals produced by the reaction are finally measured by the detector; in this scheme, the capillary acts as a nano-or microreactor as well as a microseparator. The effective capillary length can be controlled by changing the interval between metal ion and H2O2 injections. This scheme has been successfully applied to the detection of Co2+, Cu2+ Mn2+, and Vo(2+). The detectability is discussed from several viewpoints, such as the intrinsic catalyst ability of metal ions, the kinetics of the catalytic reaction, and reaction times determined by the mobility of the zone of the metal ion. Some strange behaviors, which cannot be predicted by batch experiments, are also reported.