Additional proof has been obtained for the fact that the triplet state in DNA and poly dAT resides at the thymine residue by comparing the intensities of the triplet ESR and phosphorescence signals from DNA's from various sources having different (A+T)/(G+C) ratios. The TMP triplet which is not populated from the exited singlet state in dilute neutral solutions can be populated by having the TMP present in high concentrations or through energy transfer from the triplet state of acetone or acetophenone. The triplet state produced in this manner has been characterized by its Δm=2 ESR line, its phosphorescence spectrum, its decay time, and the effect of deuterium substitution on the latter. In all these respects the DNA and poly dAT triplet resembles the neutral TMP triplet (T) more closely than it resembles the ionized (pH 12) TMP triplet (T−). A determination of the acidity constants of the excited states of A and T gives strong evidence against single-proton transfer from the excited singlet or triplet state of T to A. While this cannot be considered final proof that the DNA triplet is that of neutral thymine, this is accepted as the most likely hypothesis.