A self-cleaning spray nozzle, and in particular a self-cleaning spray nozzle for use in an apparatus for the preparation of a particulate material by a controlled agglomeration method, for example a method for controlled growth of particle size. The apparatus is especially suitable for use in the preparation of pharmaceutical compositions containing a therapeutically and/or prophylactically active substance which has a relatively low aqueous solubility and/or which is subject to chemical decomposition.