Raman microscopy has been used to characterize the interlayer anions in synthesized hydrotalcites of formula Mg6Al2(OH)16(XO4)·4H2O, where X is S, Mo or Cr. The Raman spectrum shows that both the chromate and molybdate anions are not polymerized in the hydrotalcite interlayer. This lack of polymerization is attributed to the effect of pH during synthesis. A model of bonding is proposed for the interlayer anions based upon the observation of two symmetric stretching modes and symmetry lowering of the chromate, molybdate and sulphate anions. Two types of anions are present, hydrated and hydroxyl surface-bonded. Copyright © 2005 John Wiley & Sons, Ltd.