A systematic study is reported of mixed self-assembled monolayers (SAMs) on Au(111) of alkanethiols with two compounds having chain length differences of 4-10 carbon atoms. These mixed SAMs were characterized by scanning tunneling (STM) and atomic force (AFM) microscopy. At the center of atomically flat Au(111) terraces, homogeneous mixed SAMs of octanethiol/tetradecanethiol and octanethiol/dodecanethiol were achieved by coadsorption at 50 degreesC. The surface composition of the mixed SAMs can be adjusted by varying the solution composition. These mixed SAMs did not segregate in air for at least 3 days. At: room temperature, phase segregation was observed when the chain length difference was greater than four carbons. The effects of material defects and temperature on phase segregation and preferential adsorption are discussed. Possible mechanisms are suggested for coadsorption at room and higher temperatures. This work provides an alternative approach for the preparation of homogeneous mixed SAMs at the molecular scale.