Three new gallium(III) and iron(III) complexes with L-proline-thiosemicarbazone hybrids, namely [GaCl(L-Pro-FTSC–2H)]·0.7H 2O·0.5CH 3OH (1·0.7H 2O·0.5CH 3OH), [GaCl(dm-L-Pro-FTSC–2H)]·0.4H 2O (2·0.4H 2O) and [FeCl(L-Pro-FTDA–H)]Cl (3) were synthesised and comprehensively characterised by spectroscopic methods ( 1H, 13C NMR, UV–vis), ESI mass spectrometry and X-ray crystallography. The complexes are soluble in biological media to allow for assaying their antiproliferative activity. The complexes were tested in three human cancer cell lines, namely HeLa, A549 (non-small cell lung cancer), LS174 and nontumorigenic MRC5. Complex formation equilibrium processes of L-Pro-FTSC with gallium(III), iron(II) and iron(II) ions were investigated in solution. The formation of mono-ligand iron(II) and gallium(III) complexes with pentadentate ligands and relatively low aqueous solution stability was found. Between iron(III) and the ligands, a redox reaction takes place via the oxidative cyclisation of the thiosemicarbazone.