© 2018 American Chemical Society. The elusive γ phase of trimesic acid (TMA, 1,3,5-benzenetricarboxylic acid) has been prepared by recrystallization of commercial α-TMA in acetic acid. This process yields a modified γ-TMA phase that contains guest acetic acid solvent molecules with an approximate stoichiometry of γ-TMA·1HAc. According to 1H NMR and crystal structure determination, guest molecules are located both in the channels and in relatively isolated cavities. Despite the constricted connection between channels and cavities, solvent guest molecules are easily removed, even at room temperature, yielding guest-free γ-TMA. This process is reversible, since pristine γ-TMA can reabsorb acetic acid vapor at room temperature, yielding again γ-TMA·1HAc. Conversely, γ-TMA only adsorbs negligible amounts of N2 at 77 K or CO2 at 273 K, denoting that the guest-adsorbent interaction is a key factor governing adsorption.