The compound Cu2Sb is known as a promising anode material for the lithium-ion battery due to its good cycle performance and large volumetric capacity. Our previous study revealed the existence of a LixCuSb-type solid-solution phase with a Heusler-type structure formed during the Li uptake into the Cu2Sb phase, and we suggested that this Heusler phase would play an important role in promoting low irreversibility and a long cycling life. Therefore, this paper details a computational study of phase stability in LixCuSb (0 = x = 2) from the viewpoint of the Li/vacancy arrangement. The first-principles approach was carried out using the cluster expansion technique which combines density functional and Monte Carlo computations. The calculations confirmed the formation of a solid solution of LixCuSb at 300 K over the entire composition regime for x ranging from 0 to 2. In addition, there was a strong tendency for Li ordering at x = 1 where all the octahedral sites were occupied by Li ions. Combining these results with those of the previous calculations for Li-Cu-Sb ternary compounds, we can summarize the lithium insertion reaction into Cu2Sb compounds as constituting the following three steps: (i) Li uptake into Cu2Sb results in the formation of LiCuSb and Cu metal as a three-phase coexistence reaction, (ii) solid solution formation of LixCuSb takes place for x ranging from 1 to similar to 1.6, and then (iii) Li3Sb formation accompanying Cu extrusion occurs as a three-phase coexistence reaction. (c) 2008 The Electrochemical Society.