Following the proposal of a new structural model by Fenter et al. (Science 1994, 266, 1216), several reports have indicated the existence of dimers in a self-assembled monolayer (SAM) film on Au (111). We synthesized an asymmetric disulfide with hydrocarbon and fluorocarbon chains, and observed phase-separated domains in the SAM film with force microscopy after annealing at 100 degrees C for 8 h. The phase separation clearly shows the cleavage of the S-S bond of the disulfide in the film. Although it cannot be confirmed whether the phase-separated domains consist of exchanged dimers or monomers (thiolates), we obtained new insights into the stability and diffusion of molecules in SAM films.