ABSTRACT: The structures of the four isomeric compounds of 6-methyl-quinoline with chloro- and nitro-substituted benzoic acids, C7H4ClNO4·C10H9N, namely, 2-chloro-4-nitro-benzoic acid-6-methyl-quinoline (1/1), (I), 2-chloro-5-nitro-benzoic acid-6-methyl-quinoline (1/1), (II), 3-chloro-2-nitro-benzoic acid-6-methyl-quinoline (1/1), (III), and 4-chloro-2-nitro-benzoic acid-6-methyl-quinoline (1/1), (IV), have been determined at 185-190?K. In each compound, the acid and base mol-ecules are linked by a short hydrogen bond between a carboxyl O atom and an N atom of the base. The O?N distances are 2.5452?(12), 2.6569?(13), 2.5640?(17) and 2.514?(2)?Å, respectively, for compounds (I)-(IV). In the hydrogen-bonded acid-base units of (I), (III) and (IV), the H atoms are each disordered over two positions with O site:N site occupancies of 0.65?(3):0.35?(3), 0.59?(4):0.41?(4) and 0.48?(5):0.52?(5), respectively, for (I), (III) and (IV). The H atom in the hydrogen-bonded unit of (II) is located at the O-atom site. In all of the crystals of (I)-(IV), ?-? inter-actions between the quinoline ring system and the benzene ring of the acid mol-ecule are observed. In addition, a ?-? inter-action between the benzene rings of adjacent acid mol-ecules and a C-H?O hydrogen bond are observed in the crystal of (I), and C-H?O hydrogen bonds and O?Cl contacts occur in the crystals of (III) and (IV). These inter-molecular inter-actions connect the acid and base mol-ecules, forming a layer structure parallel to the bc plane in (I), a column along the a-axis direction in (II), a layer parallel to the ab plane in (III) and a three-dimensional network in (IV). Hirshfeld surfaces for the title compounds mapped over d norm and shape index were generated to visualize the weak inter-molecular inter-actions.