The study of magneto-optical as well as transport and magnetic properties of ferrimagnetic amorphous films of Gd-Fe is extended by comparing them to single magnetic component alloys such as Fe-Bi and Gd-Au. We show that the Faraday rotation θ1 can be expressed as an additive property in terms of the Kundt’s constant K, where θ1= K(Fe)4πM(Fe) + K(Gd)4πM(Gd). Values of K(Fe) = 20 degs/cm-G and K(Gd)=-9 degs/cm-G are found. In addition the relation θ1= 2nθ11/λ where θ1is the Hall angle gives the index of refraction n for wave length λ, in reasonable agreement with measured values of n for Gd-Fe alloys obtained by other techniques. © 1985, IEEE. All rights reserved.