The polar magneto-optic Kerr effect of Bio.8Ca2.2Fe3.&V~.~Oq~has been measured between 0.5 and 0.24 pm. A strong magneto-optical transition is found at 0.47 pm which is not observed in Y3Fe50~2. This transition is concluded to be the origin of the anomalous large Faraday rotation of Bi"Ca3 "Fe3 /+0.~"V~.~O,5"0~2 recently reported and is interpreted as a charge transfer of an electron froxn an oxygen p orbital to an antibonding e orbital of the tetrahedral iron complex. The large rotation is attributed to a mixing of 6 and Bi orbitals.It has recently been found' that for garnets of composition Bi"Ca, , re, ,+,~V. . .~0» (QaV13lG) with 0. 5~x~1 the Faraday rotation for near infrared and visible radiation is much larger than fox' the rare-earth iron garnets like YgF650~2 (YIG). The experimental data on different crys tais indicate that the Faraday rotation in this wavelength region, where the electric dipole part is predominant over the magnetic dipole part, increases linearly with the concentration of Bi" ions on the dodecahedral sites. ' This effect is surprising since previous experiments on garnets and other magnetic ferrioxides lead to the conclusion that this electric dipole part of the Faraday rotation is mainly determined by the Fe" ions and their local oxygen surroundlDgs. The px'esence of othel ions, e.g. , dlffeleDt types of rare-earth ions or yttrium in the garnets" and orthoferrites, or Li and Mg in the spinel ferrites had little influence on this rotation. For these compounds a satisfactory expla™ nation of the strong magneto-optical transitions occuring in the ultraviolet region, which axe the origin of the Faraday rotation, can be given by using R molecular orbital model of Fe" oxygen complexes. Evidence for such an explanation, first suggested by Clogston, ' was given by the magneto-optical Kerr-effect measurements of Kahn, Pershan, and Remeika.To investigate the anomalous behavior of Ca-VBIG we have measured the spectrum of its magneto-optical transitions at energies up to 5 6V, using the polRX' Kerr effect. Flgux'6 I shows the Kex'1 x'otRtloD spectrum of CR2 28io BF63 9V~~0~2 between 0.6 p. m (2 eV) and 0.24 p, m (5.2 eV) in R saturating magnetic field. For comparison the 5.0 4.0 = 3. 0 ENERGY IEV) 2.0 03-V) LŨ 02-CL C3 0 C) i: -0.1--o2--0.5--0,6-l 0.4 FIG. 1. Kerr rotation spectra of Y3Fe50~& and Bio.aiCa2.~9V~.~Fe3.~0~2. Rotation peaks A. and B have in YIG the opposite sign compared with CaVBIQ because of a different orientation of the sublattice Inagnetizations (see text) .spectrum of YIG was also measured. The results. fox the latter crystal were in good agreement with those given previously by Kahn, Pershan, and Remeika. 4 The measurements were obtained with a Kerr spectrometer employing polarization modulation described elsewhere. 'The accuracy of the measured rotations is about 0 001'The rotation maxima in these spectra indicate approximately the energies of optical transitions which are magneto-optically active, i.e. , show a different response for left-and right-handed cix'-culRr polRrlze...