This invention relates to a method of functionalizing a powdered substrate. The method comprises the following steps, which method comprises passing a gas into a means for forming excited and/or unstable gas species, typically an atmospheric pressure plasma or the like and treating the gas such that, upon leaving said means, the gas comprises excited and/or unstable gas species which are substantially free of electric charge. The gas comprising the excited and/or unstable gas species which are substantially free of electric charge is then used to treat a powdered substrate and a functionalizing precursor in a downstream region external to the means for forming excited and/or unstable gas, wherein neither the powdered substrate nor the functionalizing precursor have been subjected to steps (i) and (ii) and wherein said functionalizing precursor is introduced simultaneously with or subsequent to introduction of the powdered substrate. Preferably the method takes place in a fluidized bed.