The structure of [4-(phenylazo)phenoxy]hexane-1-thioI (AzoC(6)) self-assembled monolayers (SAMs) on Au(111) has been investigated with scanning tunneling microscopy (STM) and Fourier transform infrared-reflection absorption spectroscopy (FTIR-RAS). The FTIR-RAS results yield a tilt angle of the molecules close to 0degrees, which significantly differs from the 30degrees tilt angle of linear n-alkanethiols. In STM images two types of domains are observed that have equal unit cell dimensions and two molecules per unit cell but show different tunneling contrasts, which is attributed to a different arrangement of the molecules within the unit cell. The relationship of the molecular lattice to the substrate lattice is found to be commensurate.