The appearance of new additional bands, at 580 and 630 nm, in the fluorescence spectra of porphyrins, upon the aging of their aqueous solutions, was investigated for four porphyrin systems: The deutero, the hemato, the hemato-derivative, and the proto. The conditions for quantitative conversion to the new spectra were sought. To that end the concentration levels investigated were adjusted to have each porphyrin system in a monomer-dominant state. The species fluorescing at 580 nm was denoted f580 and its rate of appearance was found to fit firstorder kinetics, for each porphyrin, the range of the rate constants being 0.03-0.10 hr-1. Two key issues were probed: (a) The identity of f580, the effort directed towards zincporphyrin. (b) Its origins-a contaminant unmasked, or a species newly formed, during the aging. Testing f580 for zincporphyrin properties, spectral at neutral pH, acid/base de/remetalation, EDTA competition, and chromatography gave in each case positive results, allowing the identification of f580 as zincoporphyrin. On the basis of the data obtained and the known sources for the contamination of aqueous solutions by zinc, the case is made for zinc porphyrin formation upon aging. The relevance of the phenomena to the therapeutic activities of porphyrins is discussed.