The present invention relates to a process for preparing a hydrated moisture control substance comprising the steps: (i) providing a predetermined amount by weight of humidity control substance having a moisture content lower than the moisture content desired with respect to the relative humidity of the atmosphere to be established and/or maintained within a sealable packaging material, (ii) determining the amount of water necessary in order to impart the desired moisture content to the humidity control substance, (iii) introducing said predetermined amount of the humidity control substance into a mixing apparatus, (iv) introducing said amount of water into the mixing apparatus under mixing and allowing the humidity control substance to adsorb the water to form the hydrated humidity control substance. The present invention also relates to a hydrated humidity control substance obtainable by said process and to a container comprising said hydrated humidity control substance and a pharmaceutical or diagnostic product.