Polar Kerr rotation (PKR) spectra of Bi-substituted YIG monocrystals, grown by flux method, were measured in the wave length region of 0.4 to 0.8 μm at room temperature. The natural grown surfaces, as well as surfaces newly cut across different parts of the crystals were all taken under examination. The variations in the height of the same characterizing peak near 2.7 eV reveal the imhomogeneity of the content of Bi ions, entering the dodecahedral site of the lattice. Our work demonstrates the possibility of PKR spectra as a tool of surface layer analysis, applicable in the investigation of such processes as ionic distribution, diffusion or surface segregation.