The speciation of ferriprotoporphyrin IX (Fe(III)PPIX) in aqueous and mixed aqueous-organic solvents has been investigated by UV-vis, H-1 NMR, magnetic, and diffusion measurements. Fe(III)PPIX has been found to form monomers, pi-pi dimers, mu-oxo dimers, and pi-stacked aggregates of mu-oxo dimers depending on concentration, pH, the presence of salts, temperature, and solvent identity. This highlights the complexity of the behavior of Fe(III)PPIX in solution. However, the presence or absence of the mu-oxo dimer is clearly dependent on solvent, with a series of aprotic solvents (5.64 M DMSO, acetone, DMF, THF, 2,6-lutidine) all promoting mu-oxo dimer formation at pH 10. By contrast, protic solvents (methanol, ethanol, propanol, ethylene glycol, diethylene glycol, and formamide) at the same concentration and under the same conditions give rise only to the pi-pi dimer variously mixed with monomer depending on solvent polarity. The pi-pi dimer has previously been shown to be present in purely aqueous solution. In the presence of 4.25 M NaCl in aqueous solution, on the other hand, both UV-vis spectra and diffusion measurements suggest the presence of large pi-stacked aggregates of mu-oxo dimers at pH 10. In aqueous DMSO at least, the temperature dependence of the dimerization constant shows that the process of mu-oxo dimer formation is endothermic and hence entirely entropy driven. This strongly suggests that formation of the mu-oxo dimer is driven by desolvation, with solvents that can act as both hydrogen bond donors and acceptors to the axial water/hydroxide ligand of Fe(III)PPIX preventing formation of this dimer species, while those that cannot act as hydrogen bond donors facilitate it. The findings permit prediction of the Fe(III)PPIX species present in different mixed solvent systems and in the case of aqueous DMSO at any given pH, concentration, and temperature.