Electrochemical behavior of coenzyme PQQ has been investigated in aprotic organic solvents using the trimethyl ester of PQQ (1(ox)H, trimethyl 4,5-dihydro-4,5-dioxo-1H-pyrrolo[2,3-f]quinoline-2,7,9-tricarboxylate) and its 1-methylated derivative (1(ox)Me, trimethyl 4,5-dihydro-4,5-dioxo-1-methylpyrrolo[2,3-f]quinoline-2,7,9-tricarboxylate). 1(ox)H shows a complicated cyclic voltammogram in organic media (CH2Cl2, MeCN, and DMSO); there are at least three reduction peaks and four oxidation peaks. The first irreversible reduction peak of 1(ox)H around -0.9 V vs ferrocene/ferricenium (Fc/Fc(+)) is very close to that of the reversible reduction peak of 1(ox)Me. Addition of an equimolar amount of acetic acid into the solution of 1(ox)Me results in a change of the reversible one-electron redox couple of 1(ox)Me/1(rad)Me(-) to an irreversible one as observed in 1(ox)H. This phenomenon indicates that protonation of the semiquinone radical anion causes disproportionation of the resulting neutral semiquinone 1(rad)H(2) into (1/2) 1(ox)H (quinone) and (1/2) 1(red)H(3) (quinol). In this case, 1(ox)H itself in bulk solution acts as the proton source to be converted into 1(ox)(-). This is confirmed by the observation of reversible redox peaks due to a 1(ox)(-)/1(rad)(2-) couple at -1.30 vs Fc/Fc(+) in DMSO and by the 2:1 formation of 1(ox)(-) and 1(red)H(3) in the thin layer UV-vis spectrum of 1(ox)H electrolyzed at -0.8 V Ag/Ag+. Solution ESR spectra of electrochemically generated semiquinone radical anion species, 1(rad)H(-), 1(rad)Me(-), and 1(rad)(2-), as well as the UV-vis spectrum of 1(rad)Me(-) are also reported to explore the electronic structures of the semiquinone radical anion species of coenzyme PQQ Addition of Ca2+, which is another cofactor of quinoprotein alcohol dehydrogenases, into the solution of 1(ox)Me causes ca. 0.57 V positive shift of the one-electron reduction potential of 1(ox)Me, showing the enhancement of the oxidation power of 1(ox)Me by a strong interaction between Ca2+ and 1(rad)Me(-). The ESR spectrum of 1(rad)Me(-)-Ca2+ complex is successfully detected and compared with that of the free semiquinone radical anion 1(rad)Me(-).