The remarkable magnetic properties of yttrium iron garnets (YIGs) underpin the use of these materials in a broad scope of spintronic and photonic applications. In particular, the addition of rare earth metals in the structure enhances to a great extent the magneto-optical activity, which is beneficial for the development of nonreciprocal optical devices. Exploiting the wavelength selectivity of magneto-optics, we have identified a range of frequencies at which one can unravel the individual contributions to the magnetism and gyrotropic response arising from cerium and iron. We envision that this outcome may pave the way to further experiments to assess quantitatively the effect on the optical properties of rare earth incorporation into YIG. ; This work was supported by the Spanish Government by the MAT2011-29269-C03, MAT2014-56063-C2-1-R, and the Severo Ochoa SEV-2015-0496 Projects and the Generalitat de Catalunya (2014 SGR 734 Project). B.C. acknowledges his grant FPI BES-2012-059023 and R.C. acknowledges his fellowship from CNPq - Brazil. S.G., M.O., and R.G. thank T. Brenninger and E. Zamburg for technical support. We also acknowledge the contribution of C. Rubio in preliminary structural and magnetic characterization of thin films as well as Dr. F. Sanchez for scientific guiding and discussions on materials issues. ; Peer Reviewed