Although the continuing research on blue thin film electroluminescence has led to fairly stable and efficient SrS:Ce devices, there is still a lack of fundamental knowledge about the origin of the remaining instability. The purity and stability of both the SrS host semiconductor and the Ce activator have been examined, in the starting materials as well as in the evaporated thin films. It is found that the SrS host is relatively inert, but that it is imperative to choose a moisture-insensitive Ce-based activator.