We present a new method of controlling nanoscale structures of mixed self-assembled monolayers (SAMs) on atomically flat Au(lll) surfaces by coadsorption from a solution containing asymmetric and symmetric disulfides. Our results showed that the mixed disulfides formed homogeneous SAMs, whereas the mixed thiols with similar surface compositions led to phase segregation on the Au(lll) surfaces. The disulfides were chosen so as to produce the same surface thiolates as the thiols. Nanoscale phase segregated domains were characterized and identified by atomic force/friction force microscopy (AFM/FFM). Preferential adsorption of the mixed SAMs was determined by X-ray photoelectron spectroscopy (XPS). There was less preferential adsorption and phase segregation of the mixed SAMs prepared by disulfides than thiols. The possible mechanism of the formation of the uniform mixed SAMs by coadsorption of symmetric and asymmetric disulfides is discussed.