IN a paper concerned chiefly with the results of measurements of the self-quenching of the fluorescence of solutions of aromatic hydrocarbons1, Weiss and Weil-Malherbe report a few measurements on the self-quenching of ethyl chlorophyllide solutions. They conclude from these latter results that it is unnecessary to postulate the existence of a long-lived form of excited chlorophyll. This conclusion has been shown by E. Rabinowitch2 to be inconsistent with the observed3 high quantum yields for chlorophyllsensitized photo-oxidation. More recently, the same author4 has pointed out that the value of the quenching constant obtained by Weiss and Weil-Malherbe is too large to be consistent with the number of encounters between chlorophyll molecules as calculated from kinetic theory.