For heat/cold storage systems a granular bed of salt hydrates is studied during dehydration/hydration. The water density in these beds are measured with help of NMR. Diffusion based dehydration of a granular bed of Na2SO4 center dot 10H(2)O is shown to be internally limited as larger grains dehydrate faster than smaller grains as result of cooling of the grains. In case of air flow dominated dehydration of Na2SO4.10H(2)O the supply of dry air was the limiting factor. On the other hand, for rehydration of CuCl2 with air flow through a granular bed, the absorption rate of water of the grains in the grain was the limiting factor and not the airflow. The observed limitations are not general observations for salts during dehydration and hydration, but may be different for other salts. (C) 2016 Elsevier Ltd. All rights reserved.