Scanning tunneling microscopy has been used to demonstrate phase segregation in varied composition, two-component self-assembled monolayers on gold. These monolayer films were assembled using CH3(CH2)(15)SH and CH3O2C(CH2)(15)SH, two similar alkanethiol molecules which are non-hydrogen-bonding and have identical alkyl chain lengths. X-ray photoelectron spectroscopy, infrared spectroscopy, and ellipsometry have been used to characterize the average chemical compositions and average molecular structures of these films. Scanning tunneling microscopy of single composition, self-assembled monolayers of each of these molecules shows a preponderance of defects which can be attributed to single missing chains. In mixed composition films, we observe nanometer scale molecular domains with time-dependent shapes. These observations have important implications both for the fundamental understanding of solubilities and phase segregation in quasi-two-dimensional mixtures and for applications of self-assembly in which spatial patterns of adsorbate mixtures are important.