Rapidly solidified alloy of composition Fe79Si10B10Cu1 was given a series of isochronal anneals in order to investigate changes in soft magnetic properties with the development of relatively coarse-grained nanocrystalline microstructure (∼50 nm). Results were compared with those through the development of a fine nanocrystalline microstructure (∼15 nm) in a commercial alloy of composition close to Fe75.5Si12.5B8Cu1Nb3. Results were also compared with those of alloys, Fe80Si10B10 and Fe77Si10InB10Nb3. X-ray diffraction, transmission electron microscopy, differential thermal analysis differential scanning calorimetry, magnetic force microscopy and core loss measurements were used to determine phases present, grain size distributions, impingement effects and dependence of magnetic properties on 1 h annealing temperature. Substitution of 3%Nb for 3%Fe in Fe80Si10B10 resulted in a shifting of Fe(Si) crystallisation to higher temperatures, a decrease in nucleation rate but no evidence of lowering of Fe(Si) crystal growth rates. After initial deterioration of magnetic properties in Fe79Si10B10Cu1, due to crystallisation of α-Fe(Si), improvements in properties with increasing 1 h annealing temperature were observed. These improvements were attributed to smaller particle sizes and narrower size distributions, in turn related to the combined effects of higher nucleation rates and grain impingement at the higher annealing temperatures. In both alloys a deterioration of magnetic properties at even higher temperature is clearly associated with onset of the eutectic reaction and the formation of Fe2B.