Crystallization of Fe78-xCoxSi9B13 amorphous alloys has been studied by differential scanning calorimetry (DSC), thermomagnetic gravimetry and X-ray diffraction techniques. Thermal stability of the amorphous alloy monotonically decreases with increasing cobalt content. The devitrification process as detected by DSC occurs in two main stages partially overlapping in temperature, but magnetic characterization reveals a third stage for cobalt-rich alloys. Primary precipitation of alpha-(Fe, Co) is followed by a polymorphic reaction to give (Fe, Co)(2)B phase. Fe3B-type phase is also detected at the end of the first crystallization stage for iron-rich alloys and beta-Co(Si) and Co2B phases are found in fully crystallized samples for x = 60. The influence of relaxation processes on the crystallization was investigated, but no significant effects of pre-annealing on the crystallization parameters resulted.