The first muon spin rotation study of the precipitation in Fe(Cu) alloys is presented. The precipitation was induced by a series of isochronal (1-h) annealing steps at temperatures T α between 460°C and 660°C and monitored by electron microscopy and hardness measurements; particle size and density for the early stage were also estimated by classical nucleation theory. The advance of phase separation appears in the muon signal in two ways. At the early stage, with small particles and a low precipitated solute fraction, the depolarization rate rises and peaks at T α = 540 ° C , whereas the depletion in solute of the matrix and the commencement of ripening is revealed by a rapid change in the precession frequency, going through a maximum at T α = 580 ° C . The method is sensitive at relatively low measuring temperatures (180 to 230 K). All these features could be interpreted in terms of precipitate size and density, taking into account the role of the “free diffusion length” of the implanted muons in the sample. Aspects of future application of the method in the study of neutron irradiated iron and iron-based alloys are also discussed.