Abstract: Novel compounds obtainable by reacting together an imido-reagent such as diphenylphosphonimido-triphenylphosphorane), or a phosphine oxide such as p-tolyldiphenylphosphine oxide, with a chelate of a transition or lanthanide or actinide metal, such as tris(dibenzoylmethide)europium III, have the property of fluorescing in UV radiation. The invention includes solid polymer bodies containing such compounds, or chelates of transition or lanthanide or actinide metals generally, the bodies having the property of emitting light or infra-red radiation when subjected to a flux of ultra-violet or ionising radiation by virtue of internally generated, e.g. by tritium ionising radiation. The body is preferably of polystyrene formed by polymerising the monomer in the presence of the compound or metal chelate.