Fluorescence decay dynamics and structure of a meso-tetrakis(p-sulfonatophenyl)porphyrin, referred to herein as either H2TSPP4- or TSPP, which aggregates in highly acidic homogeneous solution, are discussed. The fluorescence Lifetime of the aggregate is found to depend on whether electronic excitation is to the B (Sorer) slate or the Q state. This finding is interpreted as indicating that the aggregate’s effective size differs in the two cases. Also, a discussion is provided of possible nonradiative pathways that might affect measured fluorescence lifetimes. Specific structural changes that occur within the porphyrin upon N-protonation and incorporation within the aggregate are also discussed. Additionally, vibrational frequencies are assigned to specific bonds of the porphinato macrocycle, and comparisons are made between bond lengths in the aggregate environment and those of the monomeric dianion.