A processor for developing diazo film defined by a pair of flat platens disposed within a housing and spaced apart a distance only slightly greater than the thickness of the film. The housing includes intake and outlet openings aligned with the space between the platens and means for advancing an incoming film from the intake opening, through the space between the platens and for discharging it through the outlet opening. The platen facing the emulsion side of the film is heated and includes at least one passage through which a metered amount of aqueous ammonia is passed for each film that is to be developed. The ammonia is vaporized in the passage and discharged against the emulsion side of the film. A transverse groove in the emulsion facing surface of the platen communicates with the passage to distribute the ammonia vapor over the full width of the film. The developing temperature is between about 150.degree.-200.degree. F., the ammonia vapor pressure does not substantially exceed atmospheric pressure and developing times are no more than a few seconds.