A stabilized storage phosphor is disclosed intended to be exposed to X-radiation and subsequently stimulated to emit light is disclosed. The storage phosphor is the product of firing a combination of species characterized by the relationship:MFX.sub.1-z I.sub.z.uM.sup.a X.sup.a :yA:eQ:tDwherein M is an alkaline earth; X is selected from the group consisting of Cl and Br; M.sup.a is an alkali; X.sup.a is a halide; A is selected from the rare earths Eu, Ce, Sm and Tb; Q is selected from among metal oxides; D is selected from elements 23 to 28 of the periodic table; z is from 1.times.10.sup.-4 to 1; u is from 0 to 1; y is from 1.times.10.sup.-4 to 0.1; e is from 0 to 1; and t is from 0 to 10.sup.-2. The storage phosphor is stabilized against decline of photostimulated luminescence intensities by the inclusion of a monocyclic compound having a molecular weight of less than 350, exhibiting both acid and base dissociation constants of less than 10.sup.-5 at 25.degree. C. and containing the following ring atoms: ##STR1## where N.sup.T represents a tertiary amine nitrogen atom, C.sup.1/2 represents primary or secondary carbon atom, O represents an oxy linkage, and Z represents a linking group providing two ring carbon atoms to complete a five-membered ring, An oxysulfur reducing can be employed in combination. The stabilized storage phosphor is useful in storage phosphor screens used to record imagewise exposures to X-radiation.