We reported magnetooptical properties of Eu3+(4t(6)) and Tb3+(4f(8)) in single crystals of Gd3GasO12 (GGG),Y3Ga5O12 (YGG),and Eu3+(4f(6)) in Eu3Ga5O12 (EuGG) for both ions occupying sites of D2 symmetry in the garnet structure.Absorption,luminescence,and magnetic circular polarization of luminescence (MCPL) spectra of Tb3+ in GGG and YGG and absorption and magnetic circular dichroism (MCD) of Eu3+ in EuGG were studied.The data were obtained at 85 K and room temperature (RT).Magnetic susceptibility of Eu3+ in EuGG was also measured between 85 K and RT.The magnetooptical and magnetic susceptibility data were modeled using the wavefunctions of the crystal-field split energy (Stark) levels of Eu3+ and Tb3+ occupying D2 sites in the same garnets.The results reported gave a precise determination of these Stark level assignments and confirmed the symmetry labels (irreducible representations) of the closely-spaced Stark levels (quasi-doublets) found in the 5D1 (Eu3+) and 5D4 (Tb3+) multiplets.Ultraviolet (UV) excitation (＜300 nm) of the 6PJ and 6IJ states of Gd3+ in the doped GGG crystals led to emission from 5D4 (Tb3+) and 5D1 and 5D0 (Eu3+) through radiationless energy transfer to the 4f(n-l)Sdband of Tb3+ and to UV quintet states of Eu3+.The temperature-dependent emission line shapes and line shifts of the magnetooptical transitions excited by UV radiation suggested a novel way to explore energy transfer mechanisms in this rare-earth doped garnet system.