Three new lanthanide complexes with dihalo-substituted 8-quinolinol: [Gd(BrQ)₃(H₂O)₂]·1.33EtOH·0.33H₂O (1), [Dy(ClQ)₃(H₂O)₂]·1.33EtOH·0.33H₂O (2), [Er(ClQ)₃(H₂O)₂]·1.33EtOH·0.33H₂O (3) (H-BrQ = 5,7-dibromo-8-quinolinol, H-ClQ = 5,7-dichloro-8-quinolinol) have been synthesized and characterized by elemental analysis, IR, ESI-MS, single-crystal X-ray diffraction and TGA. The single-crystal X-ray diffraction analyses reveal that complexes 1–3 are mononuclear and isostructural. Each metal centre is coordinated by three dihalo-substituted 8-quinolinol and two aqua ligands. The in vitro cytotoxicity of dihalo-substituted 8-quinolinol and complexes 1–3 against liver cancer BEL7404 was evaluated. The IC₅₀ values of 1–3 to BEL7404 were 47.2 ± 2.6, 18.3 ± 1.0 and 31.5 ± 1.2 nM, respectively. The lanthanide complexes 1–3 exhibited significantly enhanced cytotoxicity vs. free substituted 8-quinolinol. The binding properties of dihalo-substituted 8-quinolinol and complexes 1 and 2 to DNA were investigated by UV-vis, fluorescence, CD spectroscopy and viscosity measurements, as well as agarose gel electrophoresis experiments. Both complexes 1 and 2 interact more strongly with DNA than the free quinolinol ligand. Intercalation is the most probable binding mode for both the complexes and the quinolinol ligands.