The temperature (4.5-700 K) and magnetic field (0-50 kOe) dependencies of the dc magnetization, and ac magnetic susceptibility in 5 Oe, 125 Hz ac field of polycrystalline Tb(Mn[Subscript l-x]Fe[Subscript x])₂ alloys with x = 0.35, 0.5, and 0.65 have been measured. The substitution of Fe for Mn introduces a negative chemical pressure in the cubic C-15 crystal structure, decreasing the lattice parameters and the distance between Mn atoms below the critical value of 2.66 Ã Â . At 5 K, all studied Tb(Mn[Subscript l-x]Fe[Subscript x])₂ alloys exhibit ferrimagnetism which is created by the antiparallel alignment of the magnetic moments of Tb and Fe atoms. The Curie temperature, the remanence and coercivity increase with x increasing. For all alloys a change of the slope of the plot of the inverse dc magnetic susceptibility vs. temperature was observed, and the corresponding temperature increases with increasing x, i.e. with decreasing lattice parameters. The temperature dependence of the ac magnetic susceptibility of all of the Tb(Mn[Subscript l-x]Fe[Subscript x])₂ alloys exhibits a peak at Curie temperature which is typically observed for ferromagnetic materials with non-zero coercivity and remanence. For polycrystal samples, the diagonal optical conductivity ([Sigma subscript lxx]) has been measured using ellipsometry at zero magnetic field. The magneto-optical Kerr rotation angle and ellipticity have been measured by Kerr spectrometry at room temperature and a magnetic field of 5 kOe, the results show that Tb(Mn₀₅̣Fe₀₅̣)₂ and Tb(Mn₀₃̣₅Fe₀₆̣₅)₂ have larger Kerr rotational angle than TbFe₂ at room temperature. The magneto-optical measurements verified the conclusion that spin-orbit interaction and exchange splitting of the electronic structure are the dominant factors which determine the magneto-optical properties of the R-TM system.