The subsolidus phase diagram of the system La 2O 3SrOCuO at 950°C under 1 bar of pure oxygen has been investigated and a new ternary compound, La 1+ xSr 2- xCu 2O 5.5+δ with 0.05 ≤ x ≤ 0.15, was isolated. This compound crystallizes in an orthorhombic unit cell with lattice constants related to the lattice constant of the perovskite cubic unit cell, ap, by a = 3.80 Å ≅ ap, b = 11.48 Å ≅ 3 ap, and c = 20.23 Å ≅ 5 ap. The structure is isotypical to that of LnSr 2Cu 2O 5.5+ δ with Ln = Sm, Eu, or Gd. Reported data on the crystal chemistry of the equilibrium compounds in the system La 2O 3SrOCuO have been summarized and compared with the present data. The structure of all compounds is built up of a LaO rock-salt layer separated by a number of LaCuO 3 perovskite layers. The general formula is (LaO)(LaCuO 3) n where La can be replaced either partly or completely by Sr. Compounds are found for n = 1, 2, and ∞. The structures of the compounds show different types of oxygen vacancy ordering.