Dehydration of NdCl3 . 6H(2)O, TbCl3 . 6H(2)O and DyCl3 . 6H(2)O was carried out by fluidization in a gas mixture of Ar and HCl. In a series of isothermal experiments the effect of different process parameters was considered. Water vapour pressure of gas leaving the fluidized bed was estimated by using a simple mass balance. It was shown that the rate-limiting step of the overall dehydration process in the fluidized bed is removal of water vapour by a saturated bulk gas how. However, the saturated gas phase data could not be thermodynamically interpreted. The reason to this is discussed.