The magneto-optical (MO) Kerr effect in CosPt1−s alloys follows from the permittivity tensor component ɛxy times the function v(N) of the refractive index, N. Re(ɛxy) displays a maximum at the photon energy ℏω ≈ 1 eV, |v[N(ℏω)]| increases 30 × from 1 to 6 eV. Re(ɛxy) creates a low ℏω peak in polar Kerr rotation (PKR). The PKR peak above 3.6 eV results from v[N(ℏω)] that modulates Re(ɛxy). The Re(ɛxy) maximum reduces with s while |v[N(ℏω)]| above 4 eV becomes steeper. PKR peak at ℏω > 3.6 eV originate mostly from v(N). The mechanism affects MO effects in other ferromagnetic metals and alloys.