Hydrotalcites with cationic ratios of 2:1, 3:1, and 4:1 were synthesised using the co-precipitation method. The mechanism of inclusion of arsenate, vanadate, and molybdate into these structures is investigated using the combination of X-ray diffraction, Raman spectroscopy, and thermal analysis. Results show that hydrotalcites with cationic ratios of 3:1 are thermally more stable then the 2:1 and 4:1 structures. The increase in thermal stability of the 3:1 hydrotalcite structures is understood to be due to the intercalation of arsenate, vanadate, or molybdate, by an increase in hydrogen bonds associated with the intercalated anion. The 3:1 vanadate hydrotalcite is the most thermally stable hydrotalcite investigated. It is observed that the predominant mechanism for inclusion of the three anionic species is adsorption for 2:1 and 4:1 hydrotalcites, and intercalation for the 3:1 hydrotalcite structures. The intercalation of arsenate, vanadate, and molybdate into the hydrotalcite structure increased the interlayer distance of the hydrotalcite by 0.14, 0.13, and 0.26 A. respectively. (C) 2008 Elsevier Inc. All rights reserved.