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Coadsorption of two different carboxylic acids, benzenetribenzoic acid and trimesic acid, was studied at the liquid−solid interface in two different solvents (heptanoic and nonanoic acid). Independent alteration of both concentrations in binary solutions resulted in six nondensely packed monolayer phases with different structures and stoichiometries, as revealed by means of scanning tunneling microscopy (STM). All of these structures are stabilized by intermolecular hydrogen bonding between the carboxylic acid functional groups. Moreover, phase transitions of the monolayer structures, accompanied by an alteration of the size and shape of cavity voids in the 2D molecular assembly, could be achieved by in situ dilution. The emergence of the various phases could be described by a simple thermodynamic model.