Stabilisation against water uptake of hygroscopic salts useful as catalysts by formation of anhydrous adducts with particulate support materials in order to facilitate manipulation and storage. The adducts comprise a particulate support material and a hygroscopic salt of a species of formula R sR sR sR sN, R sR sR sR sP, R sR sR sR sAs, wherein R1-R4 which may be identical or different represent hydrogen or alkyl or aryl groups, or of Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr, Ba, Cu, Ag, Zn, Cd, Au, Hg, Ga, In, Tl, Sn, Sb or Pb, the salt anion being fluoride, bromide, chloride, sulphate, nitrate, nitrite, cyanate, thiocyanate, peroxide, hydroxide, acetate, formate or comprises phosphorus and oxygen, the salt being coated on the support material thereby being rendered less hygroscopic; provided that when the species is Na, the anion is other than hydroxide and when the species is Tl the anion is fluoride, bromide or chloride. Adducts of salts useful in catalytic fluorination, e.g. R sR sR sR sN sF s and CsF are of especial interest.