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The delayed fluorescence found in rigid glass solutions of cyanine dyes is shown to result from the presence of trace quantities of phosphorescent impurities and not from a low energy photoionization process, as previously suggested. The mechanism of the delayed fluorescence is dye absorption of the impurity phosphorescence and subsequent dye fluorescence. For quinoline cyanine dyes, the predominant impurities are identified as quinolones, highly phosphorescent compounds formed by oxidative degradation of the dyes.