The theoretical equations describing the reversible couple for double potential-step chronoamperometry and chronocoulometry at a microdisk electrode are derived. The experimental chronoamperometric and chronocoulometric responses at a platinum microdisk electrode with radius 10 mu m in the ferrocyanide/ferricyanide system for the time range of current measurement from 300 mu s to 500 ms agree well with the theory and demonstrate the advantages of the ultramicroelectrodes in electrochemical transient study.