AbstractNanocrystalline alloys present a new class of materials that exhibit interesting magnetic properties. These stem primarily from the presence of crystalline grains with the size of several tens of nanometers embedded in a residual amorphous matrix. In this paper, we employ positron annihilation spectrometry to investigate the relation between positron lifetime and microstructure of the NANOPERM‐type metallic Fe76Mo8Cu1B15 alloy. The behaviour of positron lifetime with respect to the annealing temperature is discussed for samples, which were prepared by one hour thermal treatment in a vacuum at temperatures ranging from 330 up to 700 °C. Samples in amorphous (as‐quenched) state were analysed, too. The results of this work confirm that the optimum annealing temperature of 550 °C correlates with the minimum of free volume and coincide with the temperature at which superior magnetic properties are observed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)