Abstract Barium titanyl oxalate tetrahydrate, Ba(TiO)(C 2 O 4 ) 2 .4H 2 O, has been investigated by means of X-ray powder diffraction. Precise powder diffraction data were obtained by a conventional diffractometer with strictly monochromatic radiation. Unit cell dimensions were determined by an indexing program based on the variation of parameters by successive dichotomies. A monoclinic cell was found, a=14.044(2)Å, b=13.812(2)Å, c=13.382(2)Å, β =91.48(1); V=2594.9Å 3 , which is characterized by the figures of merit M 20 =46.5 and F 30 =107(0.0056, 50). The complete powder pattern was reviewed by means of the program NBS*AIDS83 and the 81 first lines were indexed. Structural imperfections were not detected from the diffraction line widths, which are comparable to the instrumental resolution.