A capillary electrophoresis (CE)/electrochemical detection system with on-line deoxygenation was developed, consisting of a deoxygenation injector, a deoxygenation protector, and an electrochemical detection cell. When the system was utilized for 60 min, the steady-state current of oxygen detected could be dropped to 3% of the original value for the gold/mercury amalgam electrode and to 8% of the original Value for the gold electrode, and the limit of detection could be decreased two orders of magnitude for the reducible analytes such as Tl+ (from 3.1 x 10(-5) mol/L to 8.0 x 10(-7) mol/L) and metronidazole (from 3.8 x 10(-5) mol/L to 4.0 x 10(-7) mol/L).