The optical and magneto-optical constants of several magnetically soft thin film alloys have been determined for λ=6328Å using the transverse Kerr effect and ellipsometric techniques. The coercive force and magnetization were also measured. The alloy systems Fe1-xBx (0.15?x?0.5) and Fe1-xSix (0.1?x?0.3) and iron-overcoated permalloys were studied as were Fe, Co and permalloy for reference. When used as transverse Kerr effect mirrors for ring laser gyros, the Fe-rich Fe-Si alloys and iron-overcoated permalloys have the best calculated performance.