X-ray diffraction and cross-sectional transmission electron microscopy were used to investigate the structure of ZnS:Mn films used for direct-current thin film electroluminescent devices. A particular aim was to identify the cause of localized destructive breakdown events, a failure mode to which these devices are prone. All the material prepared in this work proved to be polycrystalline with a columnar microstructure, in the cubic phase with preferred orientation in the 〈111〉 direction. Subtle differences were found, however, when deposition parameters were varied. The effects of substrate materials, growth atmosphere and growth temperature were studied, and also the effects of annealing the films. The observation of minute cavities in ZnS deposited in an Ar-H 2 mixture, and the migration of these cavities to the grain boundaries on annealing, are considered to be particularly significant results. So, too, is the suppression of preferred orientation in films deposited in Ar-O 2 mixtures. However, although a partial correlation has been achieved, it is concluded that the failure of ZnS thin films by localized destructive breakdown is not uniquely related to structure; the relationship between fundamental electrical properties and structure and composition must also be established.