The authors reported that θ' phases heterogeneously precipitate on small black spots formed at the just early stage of high temperature aging of a ternary Al-Cu-Sn alloy resulting in the aging acceleration. The present work was undertaken to make clear whether the black spots are really Sn-rich phases or not by means of a energy dispersive X-ray spectroscope mounted on a transmission electron microscope. The black spots are proved to consist of Sn atoms as predicted by the authors. Sn atoms are not absorbed at the interface between θ' and matrix reversely to the theory by Hardy et al. The Sn-rich phases will play an important role even in precipitation of θ" or G.P. zones.