A thin film phosphor for an electroluminescent device, the phosphor being selected from the group consisting of thioaluminates, thiogallates and thioindates having at least one cation selected from elements of Groups IIA and IIB of the Periodic Table of Elements. The phosphor being activated by a rare earth metal and containing oxygen as a partial substitute for a portion of the sulphur in the crystal lattice of the thiogallate, thioindate or thioaluminate. The phosphor is a single phase homogeneous compound and provides improved luminance stability. An electroluminescent device comprising the thin film phosphor is also described and methods of making the phosphor of the invention.