The self-assemblies of di-acids HOOC-(CH2)(n)-COOH (n = 20, 18, 16, 14, 12, 10) in three solvents hexanoic acid, octanoic acid, and decanoic acid on highly oriented pyrolytic graphite ( HOPG) were studied with scanning tunneling microscopy (STM). In the solvent hexanoic acid, solvent molecules coadsorb with HOOC-(CH2)(n)-COOH (n = 20, 18, 16) via formation of hydrogen bonds. The solvent octanoic acid coadsorbs with HOOC(CH2)(n)-COOH (n = 20, 18). Decanoic acid only coadsorbs with HOOC-(CH2)(20)-COOH. In each solvent, the trend of coadsorption between solvent molecules and di-acid molecules decreases with decreasing chain-length of di-acid molecules. These systematic investigations suggest that coadsorption of solvent molecules with di-acid solute molecules is mainly dependent on the relative hydrogen-bond densities in the formed monolayer. This is consistent with the maximization of adsorption heat of the self-assembled monolayers of di-acids dissolved in solvents of carboxylic acids.