A thin film phosphor for an electroluminescent device, the phosphor being selected from the group consisting of thiasluminates, 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 a Group IIIB element as a partial substitute for a portion of aluminum, gallium or indium of the thiogallate, thioindate and thioaluminate. The phosphor provides improved luminance. An electroluminescent device consisting of the thin film phosphor on a substrate is also described.