The structure of self-assembled monolayers of p-terphenyl-4-carboxylic acid and the mixed monolayers of this acid with n-hexadecanoic acid on silver surface were studied by reflection-IR spectroscopy, near-edge X-ray absorption fine structure (NEXAFS) measurement, X-ray photoemission electron microscopy (X-PEEM), and atomic force microscopy. Exposure of the p-terphenyl-4-carboxylate monolayer to H2S vapor resulted in reorganization of the film structure into clusters of the corresponding free acids, in tens of nanometer dimension. Exposure of the mixed monolayer to H2S resulted in reorganization of the mixed monolayer film into phase-separated clusters of respective component molecules. The saturated aliphatic acid formed clusters of submicrometer size, whereas the p-terphenyl-4-carboxylic acid formed clusters of tens of nanometer size, presumably due to different surface mobility and/or intermolecular interaction of the two types of molecule. Restoration of the monolayer film from the clusters, driven by the reaction between the free acid molecules and the basic surface sites, proceeded at different speeds for the two types of molecules. The saturated acid monolayer was restored much faster than the p-terphenyl-4-carboxylic acid monolayer. A domain-separated monolayer in several micrometers scale was obtained. The process was imaged by tapping mode atomic force microscopy.