In connection with the large Faraday and Kerr rotations found previously in the compounds Bi3−2xCa2xFe5−xVxO12(CaVBIG), three series of related garnets containing bismuth have been prepared and the polar Kerr rotation spectra of polycrystalline samples of these materials have been studied. Three strong rotation peaks were observed near 2.7, 4, and 5 eV. In the first series, Y2.2−2xBi0.8Ca2xFe5−xVxO12withx= 0−1.1, the influence of the V5+and tetrahedral Fe3+concentration on the three transitions was studied, in the second series, Y3−xBixFe5O12(x= 0−1.0), the influence of Bi3+concentration, and in the third series, Y2.0−xBi1.0CaxFe5−xZrxO12, the influence of octahedral Fe3+. The transitions near 2.7 and 4 eV are strongly enhanced in proportion to the Bi3+content. The transition near 2.7 eV, which is responsible for the high Faraday rotation in the visible, is proportional to the tetrahedral Fe3+content. The effect of a small mixing of the oxygen −2p, tetrahedral iron 3d, and bismuth 6pelectronic wave functions is discussed in order to explain the results. The large value of the Kerr rotations (1°) is of interest for magneto‐optic applications.