Abstract Two new forms of microcrystalline thorium arsenate have been synthesised. They have been characterised as Th(HAsO 4) 2·4H 2O and Th(HAsO 4)·2.5H 2O by chemical analysis and thermal, X-ray, IR and ion exchange studies. The interlayer distances are: Th(HAsO 4) 2·4H 2O: 8.23 Å and Th(HAsO 4) 2·2.5H 2O: 8.59 Å. However, thermally and chemically, both these exchangers are less stable than Th(HAsO 4) 2·H 2O; Th(HAsO 4) 2·2.5H 2O being more stable than Th(HAsO 4)·4H 2O. The maximum lithium ion uptake and the corresponding arsenate released, respectively are: 1.75 and 1.20 mmol/g for Th(HAsO 4) 2·4H 2O at pH 9.0 and 3.42 and 0.28 mmol/g for Th(HAsO 4) 2·2.5H 2O at pH 11.8. For both exchangers, the loss of zeolitic water as well as that released on formation of thorium pyroarsenate takes place in one step between 383 and 773 K.