Bacteriopurpurin is a macrocyclic compound at the saturation level of bacteriochlorins that was specificallysynthesized as a potential photosensitizer in photodynamic therapy. It exhibits a long-wavelength Qyabsorptionband at 843 nm, which is a necessary prerequisite for a deep tissue penetration. Magnetic resonancespectroscopic properties of bacteriopurpurin have been examined by fluorescence-detected magnetic resonance(FDMR) and electron paramagnetic resonance (EPR) in conjunction with molecular orbital calculations. Theelectronic structure of the radical cation state obtained by oxidation of bacteriopurpurin with iodine is mappedby the determination of isotropic proton hyperfine coupling constants from electron-nuclear double-resonancespectroscopy at 298 K. A tentative assignment of the couplings to the various proton positions in the moleculeis achieved by comparison of the experimental values with simulated ones based on semiempirical intermediateneglect of differential overlap calculations and density functional theory. The triplet-state electronic structureof the ground-state trans-configured bacteriopurpurin is characterized by the determination of the zero-fieldsplitting parameters which represent sensitive probes for the geometric structure of the molecule. The ¦D¦ and ¦E¦ values obtained from time-resolved EPR (24.4 and 5.4 mT) are in favorable agreement with therespective values obtained from FDMR (24.0 and 5.3 mT). By both techniques, the presence of additionalphotoexcited triplet states has been confirmed. It is assumed that they arise from higher-energy tautomers ofbacteriopurpurin. Their occurrence is discussed in terms of a photoinitiated intramolecular one-proton transferfrom either ring II or ring IV of the trans-configured ground state to ring I or ring III to form one of thecis-configured bacteriopurpurins.