Abstract: An apparatus for developing non-silver halide sensitized photo plates by exposing the plates to wetted ammonia vapor includes within a single compact housing a humidification chamber containing concentrated ammonium hydroxide, a condensation chamber or trap having one common wall with the humidification chamber and apertures therein for communicating with the humidification chamber and a developer chamber having a common wall with the condensation chamber with apertures therein for communicating therebetween. A gas dispersion tube communicates with the humidification chamber for bubbling ammonia gas through the ammonium hydroxide to produce the wetted ammonia vapor. The vapor is conveyed through the condensation chamber where liquid drops are condensed out, and then to the developer chamber where plates may be carried in a rack for exposure to the vapor. The developer chamber has a closable access opening for insertion and removal of the plates.