The benzoquinone/hydroquinone (Q/H(2)Q) redox reaction has been studied by electrochemical-scanning tunneling microscopy (EC-STM) at a Pd(lll) (root3 x root3)R30 degrees -I electrode surface in a solution that contained 10(-4) M H(2)Q in 0.05 M H2SO4; iodine-pretreatment of the Pd(111) surface was to prevent chemisorption of organic-derived species. The molecule-resolved EC-STM images indicated that: (i) at a potential where only H(2)Q is present in solution, a self-assembled(root 21 x root 21)R10.9 degrees-eta (6)-H(2)Q monolayer is produced in which the H(2)Q molecules are oriented parallel to the surface; (ii) at a potential where partial oxidation (to Q) occurs, a self-assembled (root 21 x root 21)R10.9 degrees-eta (6)-QH adlayer is generated, where QH represents quinhydrone, an equimolar mixture of Q and H(2)Q; in this structure, the Q and H(2)Q molecules are oriented vertically face-to-face, and arranged alternately along a given row, reminiscent of the crystal structure of quinhydrone, The partial oxidation-induced molecular reorientation, which is reversible, most likely arises from favorable Q-H(2)Q face-to-face interactions; that is, complete oxidation would yield only flat-oriented Q species, Unfortunately, at potentials where only Q would be present in solution, I-catalyzed corrosion of the Pd starts to occur, which leads to noise-laden EC-STM images.