Abstract The preparation of new palladium(II) and platinum(II) complexes derived from α-diphenyl ethanedione bis(thiosemicarbazone), 1, and α-diphenyl ethanedione bis(4-ethylthiosemicarbazone), 2, is described. The palladium complexes 3 and 4 and platinum complexes 5 and 6 have been characterized by elemental analyses, fast atom bombardment mass spectrometry (FAB +) and spectroscopic studies (IR, 1HNMR). The crystal and molecular structures of the dimeric cyclopalladated compound 4 and the mononuclear platinum complex 6 have been determined by single crystal X-ray diffraction. The cytotoxic activity of the free ligands and palladium and platinum complexes against human A2780 and A2780 cisR (acquired resistance to cisplatin) epithelial ovarian carcinoma cells lines is also reported. The IC 50 values for compounds 1, 5 and 6 were found to be higher than that of cisplatin but the maximum antiproliferative activity was similar. Furthermore, the compounds largely retain their activity in the A2780 cisR cell line, having a much better resistance factor than cisplatin in the pair of cell lines tested.