The formation of 6-hydroxylated o-diphenolic compounds and other intermediates during the oxidation of dopamine, dopa and 3[3,4-dihydroxyphenyl]propionic acid by NaIO4 or mushroom tyrosinase was studied using high pressure liquid chromatography with electrochemical detection. The results suggest the following : (1) acidic conditions are necessary to achieve accumulation of both dopaquinone and dopaminequinone during dopa and dopamine oxidation by NaIO4 and also seem essential in the o-quinone hydroxylation pathway; (2) accumulation of o-quinones probably results in the formation of semiquinone owing to the interaction between o-quinone species and o-diphenolic precursors; (3) quinone methide is probably the reactive intermediate involved in the formation of 6-hydroxylated o-diphenolic compounds; (4) rapid hydroxylation of dopaquinone is due to the effect of the side chain carboxy group by enhancing the release of the beta proton, thereby facilitating the tautomerization of dopaquinone to quinone methide; (5) neutral buffer promotes both the cyclization pathway of dopaquinone and dopaminequinone to form their respective aminochromes and the structural rearrangement of these aminochromes to 5,6-dihydroxyindole.