Selenium is presently considered for radiographic applications in medical imaging and non-destructive testing. Conventional detection techniques include scintillators, storage phosphors and direct converters for which selenium-based alloys are ideally suited. Fabrication techniques for detectors based on selenium involve large-scale coating operations resulting in the production of amorphous layers several hundred microns thick. The chemistry and structural properties of these layers are instrumental in the performance of the detectors. We will describe the thermodynamic properties of selenium-based alloys and show that the level of dissociation of the alloy during the coating operation is determined by the nature of the vapor species present. High detector performance can only be obtained by minimizing the amount of dissociation and by preventing crystallization of the film.