We describe the characterization, by Electrospray Ionization Mass Spectrometry (ESI-MS), of neutral species using their redox properties. These molecules were ionized by oxidation-reduction reactions either off-line, in a classical electrochemical cell, or on-line using the fact that the ES source behaves like an electrolysis cell. This study has been performed both for oxidation (positive ions) and reduction (negative ions) reactions. Limitations as to the ability of compounds to be analyzed on-line have been examined as have limitations of off-line reduction in the case of fullerenes. This study offers a new approach for the ESI-MS analysis of compounds difficult to ionize.