Abstract The ternary compound Li278(In,Ag)154 ≡ Li0.643(In,Ag)0.357 with approximated composition Li278In114Ag40 ≡ Li0.643In0.264Ag0.093 was synthesized from the pure elements in a sealed iron crucible. The crystal structure was determined by single crystal X-ray diffraction (space group F 4 ¯ 3 m , a = 20.089(2) A, R1 = 0.0408, 470 F2 values, 38 parameters) and confirmed by X-ray powder diffraction. The (4b)- and (4d)-sites are exclusively occupied by lithium atoms, and the (4a)- and (4c)-sites are occupied by statistical mixtures of (Ag + Li). All sites (4a)–(4d) with local symmetry 4 ¯ 3 m are fully occupied in contrast to the related crystal structures of Li21Si5 and Li17M4 (M = Ge, Sn, Pb). The crystal structure of the compound Li278(In,Ag)154 with approximate composition Li:In:Ag = 278:114:40 represents a new superstructure variant of the bcc W-type and is compared with the similar W-type superstructures of Li21Si5 and Li17M4.