The electrochemical properties of polyhydroxynaphthoquinone from the shells of sea-urchins Strongylocentrotus intermedius (echinochrome A) and related compounds were investigated using thin layer voltammetry on pyrolytic graphite electrodes. On the basis of experimental data pH-potential diagrams for echinochrome A and its trimethyl ether are plotted. Five different forms of echinochrome A and six forms of the trimethyl ether of echinochrome A are shown to exist over the whole range of pH studied. Equilibrium constants and formal redox potentials for all conjugated forms are obtained. Electrochemical oxidation of echinochrome A and its trimethyl ether lead to the formation of different products. Tetraketotetralin is formed in the former case and naphthodiquinone in the latter.