Structural change in the self-assembled monolayer (SAM) of bis(2-anthraquinyl)disulfide ((2-AQS)(2)) on the Au(100)-(1 x 1) surface induced by the redox reaction was demonstrated, for the first time, using in situ high-resolution scanning tunneling microscopy (STM) images. The cyclic voltammogram of 2-AQS SAM in a 0.05 M HClO4 solution exhibited a well-defined stable wave due to the redox couple of anthraquillone/hydroanthraquinone. The surface excess of 2-AQS was evaluated to be 2.8 (+/-0.3) x 10(-10) mol cm(-2). The 2-AQS SAM. in the oxidized form revealed the well-ordered (root 2 x 4 root 2)R45 degrees structure at 0.8 V vs RHE. From the cross sectional profiles of the high-resolution STM images, the anthraquinone ring in the oxidized form was suggested to be relatively perpendicular to the surface. On reduction of 2-AQS SAM at 0 V, the STM image changed dramatically. 2-AQS molecules moved on the electrode surface to give a newly developed ordered structure with large area defects.