Co(10)Cu(90) nanopowder alloys have been prepared by the sonochemical wet method. In this way, Cu/Co bimetallic nanocrystallites with average diameter of 10-20 nm, presenting a homogeneous metastable solid solution of Co in Cu, were produced. Their structural characterization by X-ray diffraction, transmission electron microscopy and inductive coupled plasma-atomic emission spectrometry techniques has been used. Temperature dependences of the sample magnetization show two characteristic (blocking) temperatures associated to the typical deviation of the zero-field cooling and field-cooling magnetization curves at T1 approximately 15 and T2 approximately 310 K, respectively. This effect can be attributed to the fact that the samples consist of either superparamagnetic and/or ferromagnetic nanoparticles of different sizes. The samples were annealed at 300 degrees C and 450 degrees C and the observed evolution of their magnetic properties was explained in relation to decomposition of the metastable Co/Cu solid solution.