A new, dry method for converting photographically recorded amplitude holograms to high diffraction efficiency phase holograms is described. The dry method involves the application of elemental bromine or chlorine. The bromine vapor bleaching of transmission holograms produced efficiencies of 70.8% in dry film and 75.5% in water swelled layers. Index modulations of 0.033 have been observed. Reflection hologram efficiencies of 73% were obtained. The stability of the bromine vapor bleached holograms to printout is superior to all previous methods.