The effect of the substitution of Fe by Co on the enhancement of glass-forming ability limits and subsequent nanocrystallization was studied in a rapidly quenched amorphous system (Fe(x)Co(y))(79)Mo(8)Cu(1)B(12) for y/x ranging from 0 to 1. The effect of Cu on nanocrystallization was investigated by comparison with Cu-free amorphous Fe(80)Mo(8)B(12). Systems partially crystallized at the surface layer were prepared for y/x = 0 using different quenching conditions. The effect of heat treatment of master alloys used for ribbon casting was also assessed. The microstructure and surface/bulk crystallization effects were analysed using transmission electron microscopy and electron and X-ray diffraction in relation to the expected enhancement of high-temperature soft magnetic properties, drastically reduced grain sizes (approximately 5 nm) and Co content. Unusual surface phenomena were observed, indicating the origin of possible nucleation sites for preferential crystallization in samples with low Co content.