The structure of (\mathrm{YSr}{2} \mathrm{AlCu}{2} \mathrm{O}{7}) has been analyzed by neutron powder diffraction techniques. This compound crystallizes with space group (P 4 / \mathrm{mm} m) symmetry and with lattice parameters (a=3.8646(1)), (c=11.1139(3) \AA). The general structural features of (\mathrm{YSr}{2} \mathrm{AiCu}{2} \mathrm{O}{7}) are similar to those found for the 123 superconductor (\mathrm{YBa}{2} \mathrm{Cu}{3} \mathrm{O}{b+x}), and in particular the sequence of layers is the same in the two materials, with (\mathrm{SrO}) substituting for (\mathrm{BaO}). The (\mathrm{Al}) ions substitute for the copper ions of the chain sites of the 123 parent structure and have tetrahedral coordination. The tetrahedra form chains propagating along the (a) and (b) axes on the basal plane of the unit cell. Thus the structure is made of short-range domains within which the local symmetry is orthorhombic. All the oxygen atoms in this compound have a disordered configuration around the specialized positions of the 123 structure. This arrangement results in tilting of the (\mathrm{Cu}{5}) pyramids and in significant distortions of the coordination polyhedra of the other cations. O 1993 Acallemic Press, inc.