Abstract: This invention concerns a method for modifying a source material used in an ALD process, a method for depositing transition metal nitride thin films by an ALD process and apparatus for use in such process. According to the present invention transition metal source materials are reduced by vaporizing a metal source material, conducting the vaporized metal source material, into a reducing zone comprising a solid reducing agent maintained at an elevated temperature. Thereafter, the metal source material is contacted with the solid or liquid reducing agent in order to convert the source material into a reduced metal compound and reaction byproducts having a sufficiently high vapor pressure for transporting in gaseous form.