Compositions are disclosed comprising (a) a metal chelate wherein the metal is selected from the group consisting of europium, terbium, dysprosium, samarium osmium and ruthenium in at least a hexacoordinated state and (b) a compound having a double bond substituted with two aryl groups, an oxygen atom and an atom selected from the group consisting of oxygen, sulfur and nitrogen wherein one of the aryl groups is electron donating with respect to the other. Such composition is preferably incorporated in a latex particulate material. Methods and kits are also disclosed for determining an analyte in a medium suspected of containing the analyte. The methods and kits employ as one component a composition as described above.