The miscibility of a binary self-assembled monolayer (SAM) of 1-undecanethiol (UDT) and 11-mercaptoundecanoic acid (MUA) on Au( 1 1 1) has been studied using voltammetry of the reductive desorption of thiol, X-ray photoelectron spectroscopy, and scanning tunneling microscopy (STM). The binary SAMs show only a single peak on a voltammogram for the reductive desorption over the entire range of the mixing ratio of the two thiols, suggesting that these two thiols having the same number of methylene units are well-mixed in the SAMs. This is in marked contrast with the phase-separated binary self-assembled SAMs where the two thiols differ both in the type of omega -terminal and in the number of methylene units in their alkyl-chain parts. Correspondingly, STM images of MUA-UDT monolayers show no distinct domain formation, unlike the phase-separated SAMs such as the one composed of S-mercaptopropionic acid and hexadecanethiol. However, MUA molecules seem to prefer the domain boundaries.