Abstract Deoxyribonucleic acid (DNA) has a relatively low phosphorescent yield. In studying the ultraviolet-activated emission of DNA, or indeed of any substance with a low quantum yield, one is faced with the following question. Is the emission really due to the DNA or is it perhaps due to an impurity with a high quantum yield? A partial answer to this question may be obtained by showing that the activation spectrum for emission is identical to the absorption spectrum. The reason that this is but a partial answer is that the energy, after absorption by DNA, may be transferred to other molecules that may be present (1). This paper describes a relatively simple procedure for showing that the activation spectrum of DNA phosphorescence is identical to its optical absorption spectrum. The method should be generally applicable to phosphorescence studies of other substances.