New soft magnetic amorphous alloys with a wide supercooled liquid region were obtained by increasing the boron content (up to 30 at.%) and by addition of cobalt (composition range between 5 and 42 at.%) in FeNbB based materials. Upon thermal treatments, the stability and the crystallization behavior of Fe52Co10Nb8B30 and Fe 22Co40Nb8B30 have been studied by using X-ray diffraction (XRD) and transmission electron microscopy (TEM). On the other hand, the Curie temperature change, Tc, of the remaining amorphous phases has been determined by thermomagnetic analysis (TMG). Sub-micrometric fcc-(Co,Fe) and fcc-(Fe,Co)21Nb2B 6 grains were observed to crystallize at a relatively high temperature (950-970 K) providing an excellent stability of the amorphous states. Due to the partitioning behavior, occurring during crystallization, the Curie temperature of the remaining amorphous structure is observed to increase in the case of the Fe52Co10Nb8B30 composition while a decrease of Tc to the lower temperatures is observed with the Fe22Co40Nb8B30 composition. © 2003 Elsevier B.V. All rights reserved.