Iron disilicide in a bulk form is practically nonmagnetic. In contrast, nanoparticles of Fe Si2 exhibit superparamagnetism with blocking temperatures ranging from 8 K (15 nm) to 34 K (55 nm). Their relatively low saturation magnetization suggests that the magnetic behavior is associated with only a small fraction of Fe ions, which have a sufficient number of other Fe as nearest neighbors. The chemical disorder is presumably induced in the formation of nanoparticles. A spin glass-type anomaly below 10 K observed in specific heat data gives a further evidence for the compositional heterogeneity. © 2007 American Institute of Physics.