Fully amorphous rods with diameters up to 2 mm diameter were obtained upon 0.5 at.% Cu addition to the Fe36Co36B19.2Si4.8Nb4 bulk metallic glass. The Cu-added glass shows a very good thermal stability but, in comparison with the Cu-free base alloy, the entire crystallization behavior is drastically changed. Upon heating, the glassy (Fe36Co36B19.2Si4.8Nb4)99.5Cu0.5 samples show two glass transitions-like events, separated by an interval of more than 100 K, in between which a bcc-(Fe,Co) solid solution is formed. The soft magnetic properties are preserved upon Cu-addition and the samples show a saturation magnetization of 1.1 T combined with less than 2 A/m coercivity. The relaxation behavior prior to crystallization, as well as the crystallization behavior, were studied by time-resolved X-ray diffraction using synchrotron radiation. It was found that both glassy alloys behave similar at temperatures below the glass transition. Irreversible structural transformations take place when approaching the glass transition and in the supercooled liquid region. © 2015 Acta Materialia Inc. Published by Elsevier Ltd.