Potentiometric titration method, by means of potassium ferricyanide as titrant and recording auto-titrator, was applied to the determination of Phenidone in photographic P.Q. developers. In acidic solution, potassium ferricyanide selectively oxidizes Phenidone without oxiding hydroquinone and sodium sulfite. Therefore, the separation of Phenidone from the other ingredients of developer by extraction, that has been needed in conventional methods, is unnecessary. The reaction is not very rapid, but thesharp potential jump in titration curve was obtained in 2N-sulf uric acid.