Carbonate-intercalated layered double hydroxides have hitherto been thought to crystallize with rhombohedral symmetry (space group R (3) over barm). This widely accepted structure model comprises positively charged metal hydroxide layers wherein the cations are disordered. However, spectroscopic studies and simple chemical considerations militate against the possibility of cation disorder. This study shows that the observed powder X-ray diffraction pattern can indeed be fit to a cation-ordered crystal with monoclinic symmetry. With use of the carbonate-intercalated layered double hydroxide of Zn and Al as an illustration, the structure of the layered double hydroxide is refined by the Rietveld method. The resulting structure (space group C2/m, a = root 3 X a(0); b = 3 X a(0); c similar to c(0)/3; beta similar to 103 degrees, where a(0) and c(0) are cell parameters of the cation-disordered structure) resolves many of the anomalies of the cation-disordered structure.