Age hardening in Cu-3.2M-0.75Si (wt.%) alloy at 723 K is studied. An immediate increase of the yield strength characterizes the aging of the alloy. This is followed by the regions of constant yield strength and further by a peak. The microstructure of the alloy was studied by means of transmission electron microscope (TEM) and X-ray diffraction. Spinodal decomposition takes place followed by nucleation of the ordering coherent (Cu Ni)(3)Si particles, further annealing coherent delta-Ni2Si nucleated within the (Cu Ni)(3)Si particle. Any change of the yield strength can be described by an adequate change of the structure in the sample. The nature of the aging curves with a "plateau" is discussed. The formulas of Ashby and Labusch can be used to explain the precipitation. (C) 2002 Elsevier Science B.V. All rights reserved.