In our previous studies, the influence of L12 modified Al2.5Cu0.5Ti and Al2.7Fe0.3Ti particles on the grain refining performance of an Al cast was studied. It has been shown that the L12 modified intermetallic compound particles act as heterogeneous nucleation sites of α-Al solidification. Since nuclei are formed on the surface of the heterogeneous nucleation site particles, only the surface region of the Al3Ti particles must be modified into an L12 structure. In this study, therefore, the formation of the Al2.5Cu0.5Ti phase by the reaction between Al3Ti and Cu is studied. For this purpose, Cu/Al3Ti diffusion couple specimens and specimens of Al3Ti particles dispersed in a Cu matrix are fabricated by a spark plasma sintering (SPS) method, and the reaction between Cu and Al3Ti during heat treatment of these specimens is studied. It is found that the L12 modified Al2.5Cu0.5Ti phase is formed at the Cu/Al3Ti interface by heat treatment.