The magnetic properties and crystallization temperatures of alloys in the ternary Fe-B-Si system are reported. The Curie temperature increases slightly on replacement of boron by silicon. This results in a sharp ridge of relatively constant room-temperature saturation magnetization extending from Fe80B20to Fe82B12Si6. The coercivity exhibits a broad minimum, both before and after stress relief annealing, in the region around Fe81B15Si4and extending at least to Fe77B13Si10. The crystallization temperature increases with increasing silicon and with decreasing iron and boron. The alloys with silicon are generally easier to prepare in the amorphous state than the binary Fe-B alloys. Thus for the highest saturation magnetization alloy combined with ease of preparation, stability, and lowest losses, the alloys between Fe81B17Si2and Fe82B12Si6are preferred.