The electrochemistry of pyrroloquinoline quinone (PQQ) was studied in solubilized and immobilized states on a Au electrode modified with a chemisorbed cystamine monolayer. An electrochemically reversible diffusion-controlled reaction (k almost-equal-to 1.7 x 10(-3) cm s-1 at pH 7.0) is observed for solubilized PQQ on the cystamine-modified electrode under acidic and neutral conditions (pH less-than-or-equal-to 7) when the surface amino groups are positively charged. However, the electrochemical reduction of PQQ is completely irreversible on a non-modified Au electrode as well as on an electrode surface modified with neutral or negatively charged groups. The cystamine monolayer on the Au electrode surface was used as a basis for the covalent immobilization of PQQ via carbodiimide coupling of the PQQ carboxylic groups with the surface amino groups. The electrochemical reaction of the immobilized PQQ was reversible over a wide pH range (pH 2-11). The PQQ modified electrodes exhibited very high stability. A surface concentration of ca. 1 x 10(-10) mol cm-2, corresponding to a monolayer, and an electron transfer rate constant k(s) of ca. 3.3 s-1 (pH 7.0) were evaluated for the PQQ-modified electrode. The total amount of immobilized PQQ can be increased dramatically if an Au electrode with a very high surface roughness is used. The redox potential E-degree of -0.125 +/- 0.003 V/SCE (pH 7.0) was obtained for both solubilized and immobilized PQQ. It is suggested that the PQQ-modified electrodes developed in this work can be used to prepare biosensors based on PQQ enzymes and to facilitate chemical reactions characteristics of PQQ itself directly on the electrode surface.