Lu4O(OH)9I·3H2O is a new member of the anion exchangeable lanthanide hydroxyanion family of materials which has been synthesised hydrothermally. Its structure comprises positively charged [Lu4O(OH)9(H2O)3](+) layers with exchangeable charge balancing iodide anions located in the interlayer gallery. It has been found to undergo facile anion exchange reactions with dicarboxylate anions such as succinate and terephthalate at room temperature but reacts less readily with disulfonate anions such as 1,5- and 2,6-naphthalenedisulfonate under the same conditions. At reaction temperatures above 200 °C the cationic inorganic framework Lu3O(OH)6I·2H2O forms instead of the layered phase.