There is described a process for the purification of an impure preparation containing fentanyl by means of a reverse phase preparative chromatography process. A chromatographic column is loaded with a stationary phase, typically a silica particle having an organic ligand bound thereto. With a loading ratio of from about 50 to about 150 the impure preparation is acidified and passed through the column. The column is eluted with typically an aqueous solution of acetonitrile and the purified fentanyl is obtained in a specified cut.