Abstract—The rate of photodynamic degradation of deoxy‐nucleic acids of various origins was followed by noting changes in their melting curves. The rate of degradation was proportional to the mole fraction of guanine residues in the nucleic acids, except for T4r+bacteriophage DNA which showed greater susceptibility than this rule would predict. The ability of dyes to act as sensitizers is correlated with their ability to form dye molecules in a metastable excited state. In addition, the binding of dyes to specific sites on the nucleic acid molecule was shown to be of great importance. Biological activity is a much more sensitive indicator of structural integrity of DNA than is the melting cu