The compound, tris-(5-methylbenzimidazole) tris-(oxalato)-aluminate (III) trihydrate, (C 8 H 9 N 2) 3 [Al(C 2 O 4) 3 ]•3H 2 O, was synthesized by slow evaporation at room temperature and characterized by single crystal X-ray diffraction and X-ray powder diffraction, infrared (IR), ultraviolet (UV-visible) spectroscopies, and thermal analysis. The results show that this complex crystallizes in the monoclinic system, space group P2 1 /c, with the mesh parameters a = 13.499(7) Å, b = 14.872(9) Å, c = 16.995(5) Å, ß = 91.44(3)˚, V = 3411(3) Å 3 and Z = 4. The formula unit is composed of tris-(oxalato)-aluminate [Al(C 2 O 4) 3 ] 3− anions, tris-(5-methylbenzimidazole) cations and three uncoordinated water molecules. The geometry of the aluminum ion is octahedral, formed by six oxygen atoms belonging to three oxalate anions serving as chelating ligands. Cohesion of the structure is ensured by intermolecular hydrogen bonds of O-H...O, N-H...O type linking ionic entities and water molecules as well as by π-π and π-π* between cycles of 5-methylbenzimidazole cations. In order to clarify the intermolecular interactions formed by the organic cations and inorganic anions, an analysis of the calculated Hirshfeld surfaces was used. The UV-Vis spectrum reveals an optical band gap width of 2.88 eV, which shows that this compound has a semiconductor material behavior.