Electrochemical behavior of coenzyme PQQ has been investigated in aprotic organic solvents using the trimethyl ester of PQQ (1(ox)H, trimethyl 4,5-dihyro-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 l(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)H2 into ( 1/4 ) 1(ox)H (quinone) and ( 1/4 ) 1(red)H3 (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)H3 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 artion 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-.