In situ scanning tunneling microscopy (STM) was employed to study the structure of adlayers of crystal violet (CV) and water-soluble porphyrin formed on a sulfur-modified Au(lll) electrode in perchloric acid solution. It was confirmed that a well-defined sulfur adlayer with (root 3 x root 3)R30 degrees structure can be prepared on Au(lll) under potential control. Highly ordered adlayers of the molecules were found to form on the sulfur-modified Au(lll) surface. In situ STM revealed the characteristic shape, packing arrangement, and even internal structure of each molecule. It is demonstrated that the sulfur-modified Au(lll) electrode can be used as a substrate suitable for the investigation of the adsorption of organic molecules in solution.