The rather complicated electrochemical behavior, in aqueous solution, exhibited by catechol(1,2-dihydroxybenzene) and its derivatives containing carboxylate and sulphonate groups, can be conveniently controlled by means of their conversion into the corresponding blue ruthenium(III)-edta complexes (edta = ethylenedinitrilotetraacetate ligand). In the free form, the catechol species undergo a primary electrochemical oxidation process in the range of 0.3-0.6 V versus SHE, yielding quinone products susceptible to a number of pH dependent, secondary chemical reactions. When coordinated to the ruthenium(III)-edta unit, their electrochemical and spectroelectrochemical behavior is dramatically changed, approaching that of metal complexes with non-innocent dioxolene ligands. In this case, the reduction of the ruthenium(III) metal ion proceeds reversibly above pH 9, in the -(0.5-0.6) V range and the oxidation process centered on the catecholate ligands becomes reversible, leading exclusively to the formation of the semiquinone species, with no evidence of complications from the secondary reactions. (C) 2000 Elsevier Science B.V. All rights reserved.