The Li–Al–Si ternary system has been investigated and four ternary phases characterized using both powder and single crystal X-ray diffraction data. LiAlSi and Li₇Al₃Si₄ crystallize in the F4̅3m, cubic system, Li₁₈Al₂Si₆ in tetragonal I4₁/amd and Li₁₅Al₃Si₆ in hexagonal P6₃/m space group. The structure of LiAlSi (a = 5.94 Å), previously reported from powder data, has been confirmed. The new compound Li₇Al₃Si₄ has been identified and, due to a higher lithium content, its cell parameter is slightly larger (a = 6.115 Å). The tetragonal cell (a = 6.179(1) Å, c = 12.199(4) Å) of Li₁₈Al₂Si₆ may be considered as a (1 × 1 × 2) supercell of the cubic one. Structural variations are strongly correlated to compositional fluctuations and atomic substitutions, in every case the SI–Al network displays some covalent character. In the Li₁₅Al₃Si₆ hexagonal cell, bonding is more covalent leading to a heterographite-like SI–Al network.