The crystal structures of the isomorphous salts M^I₆[Mo₇O₂₄],4H₂O (M = NH₄ or K) have been refined by three-dimensional X-ray diffraction methods. Unit cell dimensions of these monoclinic compounds, space group P2₁/C with Z= 4, are, ammonium salt: a= 8·3934 ± 0·0008, b= 36·1703 ± 0·0045, c= 10·4715 ± 0·0011 Å, β= 115·958°± 0·008°; and potassium salt: a= 8·15 ± 0·02, b= 35·68 ± 0·1, c= 10·30 ± 0·02 Å, β= 115·2°± 02°.

By use of multiple Weissenberg patterns, 8197 intensity data (Mo-K_α radiation) for the ammonium compound and 2178 (Cu-K_α radiation) for the potassium compound were estimated visually and used to test and refine Lindqvist's proposed structure in the space group P2₁/c. Lindqvist's structure was confirmed and the full matrix least-squares isotropic refinement led to R 0·076 (ammonium) 0·120 (potassium), with direct unambiguous location of the cations and water molecules in the potassium compound.