The crystal structure of the low-temperature oxidized form of Sr^^CaigsBi^O^j has been determined using a combination of neutron, synchrotron, and laboratory X-ray powder diffraction data. The structure is pseudo-orthorhombic; systematic absences and successful refinement indicated the true structure to be monoclinic, with space group P2,/n. Structural refinement using only neutron powder data yielded the lattice parameters a = S.38 898 (29) A, £=5.99 334(21) A, c=5.89 586(20) A, /3= 89.997(8)°, and V=296.43(3) A 3 . This compound is a distorted perovskite phase [described in the perovskite ABO 3 formula as Sr(Bi 0 7 Cao 3 )O 3 ] with ordering of the M-site cations, resulting in the formula A 2 MM'O 6 . In this ordered structure, the A sites are solely occupied by Sr, the M sites mainly by Bi, while on the M' sites Bi and Ca are distributed in an approximate ratio of 2:3. The MO 6 and M'O 6 octahedra share corners, and are tilted with respect to the neighboring layers with an angle of -15° around all three axes. The tilt system symbol is a + a~a~ according to Glazer notation. All Bi ions are in the 5 + oxidation state. © 2000 International Centre for Diffraction Data. [S0885-7156(00)00304-3]