Synthesis, structural characterization and biological activity against several human tumor cell lines of four rhenium(I) diseleno-ethers complexes: Re(CO)3Cl(PhSe(CH2)2SePh), Re(CO)3Cl(PhSe(CH2)3SePh), Re(CO)3Cl(HO2C–CH2Se(CH2)2SeCH2–CO2H) and Re(CO)3Cl(HO2C–CH2Se(CH2)3SeCH2–CO2H)