Abstract— The absorption and emission spectra, fluorescence quantum yields and lifetimes and triplet state properties of a boronated porphyrin, the tetrakiscarborane carboxylate ester of 2,4-(α,β-dihydroxyethyl) deuteroporphyrin IX (BOPP), have been determined. This compound is an alternative photodynamic therapy (PDT) agent that exhibits highly selective tumor localization, with the potential to be used in conjunction with boron neutron capture therapy. The photophysical characteristics of BOPP are similar to other porphyrins and it exhibits marked aggregation and acid-base speciation under typical physiological conditions. In particular, protonation of the porphyrin imino (-N=) nitrogens occurs in the pH 5–7 region and influences the photophysical properties. Time-resolved confocal fluorescence imaging of the intracellular distribution of BOPP in C6 glioma cells indicates distinct subcellular localization and heterogeneity of emission. The results are interpreted and discussed in terms of the possible mechanisms for cellular uptake and localization.