We report the phase separation of a self-assembled monolayer formed from a binary mixture of adsorbates, 12-decanethiol, and an amide-containing alkanethiol of similar length (3 mercapto-N-nonylpropionamide), as studied by scanning tunneling microscopy. While mixtures of n-alkanethiols of similar length (i.e., 12-decanethiol and n-dodecanethiol) show no phase separation, the introduction of a hydrogen-bonding functionality buried deep within the film induces the formation of single-component domains on the nanometer scale. Phase separation occurs at all relative compositions studied, and for these molecules maintains the same exposed terminal functionality across the entire film. In nonequimolar concentrations of adsorbates, we observe that the solution component present in greater concentration will dominate the composition of the adsorbed monolayer in super proportion to that in solution, consistent with enthalpic contributions from both the solvent and intermolecular interactions of adsorbates.