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Two NiII compounds forming zigzag chain structures through O—H⋯N hydrogen bonds between di­carb­oxy­lic acids and NiII complexes were synthesized, and their structures were determined. They are co-crystals rather than salts., Two isostructural NiII compounds, bis­{N-[1-(pyridin-2-yl-κN)eth­ylidene]pyridine-4-carbohydrazonato-κ2 N′,O}nickel(II)–2,5-di­chloro­terephthalic acid (1/1), [Ni(C13H11N4O)2](C8H4Cl2O4), and bis­{N-[1-(pyridin-2-yl-κN)eth­ylidene]pyridine-4-carbohydrazonato-κ2 N′,O}nickel(II)–2,5-di­bromo­terephthalic acid (1/1), [Ni(C13H11N4O)2](C8H4Br2O4), were synthesized and their crystal structures determined. The pair of N,N′,O-tridentate N-[1-(pyridin-2-yl-κN)eth­yl]pyridine-4-carbohydrazonate L ligands result in a cis-NiO2N4 octa­hedral coordination sphere for the metal ions. The asymmetric units consist of two half-mol­ecules of the di­carb­oxy­lic acids, which are completed by crystallographic inversion symmetry. In the respective crystals, the 2,5-di­chloro­terephthalic acid (H2Cl2TPA, 1-Cl) mol­ecules form zigzag hydrogen-bonded chains with the [Ni(L)2] mol­ecules, with the hydrogen-bond distances in 1-Br slightly longer than those in 1-Cl. The packing is consolidated by aromatic π–π stacking between the di­carb­oxy­lic acid mol­ecules and terminal pyridine rings in [Ni(L)2] and short halogen–halogen inter­actions are also observed. The qualitative prediction of the H-atom position from the C—N—C angles of the terminal pyridine rings in L and the C—O distances in the carboxyl groups show that 1-Cl and 1-Br are co-crystals rather than salts.