The effect of substituting Fe by Co on the crystallization, structural and magnetic behaviour of Fe 72-xCo xSi 4B 20Nb 4 (X = 10,20,36,50 at%) and CO 36Fe 36Si 4-yAl yB 20Nb 4 (Y = 0,1 at%) alloys prepared in the form of melt spun ribbons has been discussed. Alloys containing optimum content of cobalt = 36 at% showed consistent coercivity at elevated temperatures. This soft magnetic property was further improved with aluminium incorporation. Transmission electron microscopy (TEM) indicated that such enhancement in the properties was due to finer dispersions of (CoFe)SiAI nanoparticles in amorphous matrix. Nanocrystallisation also raised the Curie temperature of the aluminium contained alloy. © 2009 American Scientific Publishers.