The optical and magneto‐optical constants of several magnetically soft thin film alloys have been determined for &lgr;=6328A˚ 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.