Measurements of the saturation magnetization of amorphous Fe 7 8 Si 9 B 1 3 and Fe 7 6 Si 8 B 1 6 as a function of temperature and also as a function of the duration of heating the materials at selected temperatures were made. The results suggest that the crystallization of Fe 7 8 Si 9 B 1 3 takes place in two stages, whereas the one of Fe 7 6 Si 8 B 1 6 in three. Since, during the initial stage of crystallization, regions of b.c.c. Fe(Si) are formed, measurements of the saturation magnetization of the Fe a Si 1 - a (a = 1, 0.9 and 0.75) were also carried out, in order to render possible the calculation of the activation energy of the initial crystallization.