Abstract: An electrochemistry-micro electrospray ionization-mass spectrometry (EC-mESI-MS) device was developed for in situ and on-line monitoring of the electrooxidation of m-toluidine in acetonitrile-water solution. A micro capillary was employed as electrochemical cell, which was also functioned as the electrospray ionization source for mass spectrometer. Two platinum wires with different diameters were built in the micro capillary and used as the working electrode (WE) and the counter electrode(CE), respectively. A customized AutoLab connected to the WE and CE in order to achieve the electrochemical reaction of m-toluidine and meanwhile to generate the electric field for spraying out the products from the tip of micro capillary. The applied voltage was 2.0 kV+ΔE for WE and was 2.0 kV for CE, and 2.0 kV could be regarded as the spray volage while ΔE was electrooxidation potential of m-toluidine. Based on the EC-mESI-MS technique, the electrooxidation process of m-toluidine was monitored by mass spectrometer in real-time. Prior to the MS study, the oxidation potentials of 0.1 mmol/L m-toluidine in different pH environments including pH 2, 4, 7 and 10 were determined by cyclic voltammetry. The lowest oxidation potential was observed in pH 7 at 0.17 V while the highest one was found in pH 10 at 0.84 V. Based on the obtained oxidation potentials, the corresponding reaction products were determined by EC-mESI-MS subsequently. Under the constant potential of 0.39 V, a series of oxidation products of 0.1 mmol/L m-toluidine in acetonitrile-water solution (pH 7) were detected by mass spectrometer, including radical cation, dimer, trimer, tetramer and pentamer. Since the reaction was carried out in a certain volume of solution, the polymer chain growth of the oligomer had been recorded as well. The tandem mass spectrometry (MS2) experiments indicated that one kind of oligomer had two different structures, i.e, quinone ring and benzene ring. The formation of quinone ring was due to that the benzene ring lost two protons during the reaction. By collecting the above results, the initial electrooxidation mechanism of m-toluidine was deduced. In addition, it was found that the pH of the solution had a great effect on the electrooxidation efficiency of m-toluidine. The acidic environment will inhibit the polymer chain propagation, and the stronger the acid, the more obvious the inhibition effect. Under alkaline condition, it led to a large number of dimers with quinone structure formed, and thus hinder the generation of trimer. The suggested environment is pH 7, where the large oligomer of m-toluidine can be easily formed at a low oxidation potential.