The isothermal decomposition processes of dilute Cu-Co bulk alloys have been investigated by means of analytical transmission electron microscopy. The precipitation of ferromagnetic Co particles in non-magnetic Cu matrix after aging treatments was revealed by a variety of electron-microscopy techniques: TEM diffraction contrast, high resolution electron microscopy, annular dark field STEM, energy filtered imaging and electron holography. Homogeneous spherical particles ranging from 10 to 40 nm in diameter were consistently observed from the early stages of precipitation. Precipitates grow monotonic upon aging, the lattice misfit energy between Co particles and Cu matrix increases significantly, which can further result in coherency loss of the precipitate/matrix. The precipitation of homogeneous incoherent particles was a result of double aging treatment, forming well developed and faceted crystals. In the heterogeneous decomposition mode, the kinetics of discontinuous precipitation colonies consisting of regularly-spaced rod-like particles aligned perpendicular to the grain boundary reaction front was studied. Upon further growth, these rods break down into chains of aligned particles. Intergranular diffusion models of lamellar discontinuous precipitation were found to be inadequate to apply in this alloy system. However, calculated grain boundary diffusivity indicated values two orders of magnitude higher than lattice diffusivity in this system. Magnetic measurements indicate a phase transformation from super paramagnetic to ferromagnetic, and the temperature of this transformation increases with the alloy Co composition.