Phase separation behaviors of a quenched Cu-3.0at%Ti alloy, as well as crystallographic structures of Cu-20.7at%Ti alloy have been studied using transmission electron microscopy. The furnacecooled Cu-20.7at%Ti alloy are composed of a-Cu4Ti (Ni4Mo-type) and b-Cu4Ti (Au4Zr-type) with the orientation relationship of (011)a//(110)b, a//b. As-quenched Cu-3.0at%Ti alloy showed a modulated structure with the modulation length of about 4 nm. When aged at 723K for 8 hr, the a-Cu4Ti phase emerges within the modulated or tweed-like microstructure. Prolonged aging results in the growth of the a-Cu4Ti particles and the loss of coherency. It is likely that asquenched Cu-3.0at%Ti alloy decomposes spinodally at 723K, followed by polymorphous ordering; though the present study did not exclude, as an alternative path, a decomposition mechanism based on the catastrophic nucleation.