Measurements were made on a series of glassy iron–boron alloys to determine the association of the eutectic composition (lowest melting point) and the field‐induced magnetic anisotropy. When the boron was increased from 15 to 22 at. %, the magnetic anisotropy was found to increase from 500 to 1300 J/m3. There was no sharp discontinuity in the vicinity of the eutectic. It appears that deviation from the eutectic does not greatly influence the value of the magnetic anisotropy, provided the samples are small and are drastically quenched. Specimens having the eutectic composition had the most reproducible values of anisotropy. The substitution of silicon for some of the boron in these alloys raises their Curie and crystallization temperatures. This makes possible a greater range of heat treating temperatures and permits better stress relief in the alloys. Also, the substitution of silicon permits better control of the magnetic anisotropy. This control may be a factor in obtaining improved magnetic properties since both low coercive force and low hysteresis loss in these materials depend upon obtaining the proper value of uniaxial anisotropy. Compositions that had values of the field‐induced anisotropy of 1000 J/m3had low values of core loss when properly heat treated.