The microstructure of ac‐thin film electroluminescent devices was studied by transmission electron microscopy (TEM) of cross‐sectional specimens, and was correlated to electro‐optical characteristics of the devices. The cross sections reveal the microstructure of the BaTiO3/ZnS:Mn/BaTiO3layer structure as a function of film depth. The rf‐sputtered BaTiO3films are amorphous; the electron‐beam evaporated ZnS films are polycrystalline. The first‐to‐grow region of the ZnS layers always exhibits a very small grain size. With increasing film thickness the growth of larger columnar grains is observed. At higher substrate temperatures the grains have a more conical shape. The Mn‐concentration, measured by x‐ray microanalysis on the cross‐sectional specimens in a scanning TEM, shows an overall increase with growing ZnS film thickness, because the ZnS was evaporated from a single ZnS:Mn source. Annealing at 550 °C improves the electro‐optic characteristics of the samples considerably, without changes in microstructure being observable. Annealing at 850 °C induces strong recrystallization. The initial fine‐grained region disappears and the grains extend from the bottom to the top of the ZnS film.