Dopamine [DA]+ (m/z 154), DA dimer [2DA-H]+ (m/z 307) and DA quinone [DAQ]+ (m/z 152) are detected in positive ion mode electrospray ionization mass spectrometry (ESI MS) of dopamine in 50/1/49 (vol%) water/acetic acid/methanol. H/D exchange experiments support a covalent structure of DA dimer. Thus, ESI of DA may involve 1e−, 1H+ oxidation processes followed by rapid radical dimerization. The DA quinone signal is low in ESI MS, which indicates a low efficiency of the 2e−, 2H+ oxidation reaction. On-line electrochemistry ESI MS (EC/ESI MS) with low electrochemical cell voltage floated on high ES voltage increases electrospray current and improves sensitivity for DA. The DA quinone signal increases and DA dimer signal decreases. A new configuration of the ESI MS instrument with a cone-shaped capillary inlet significantly enhanced sensitivity of ESI and EC/ESI MS measurements. A DA quinone-cysteine adduct [DAQ+Cys]+ was detected in solutions of DA with cysteine (Cys). ESI MS and EC/ESI MS indicate formation of the DA quinone-cysteine adduct by 1e− pathway. Oxidation pathways in ESI MS are relevant to biological reactivity of DA and Cys.