Gaseous iron protoporphyrin IX (heme) ions, Fe(PP-IX)(+), obtained by electrospray ionization of a methanol solution of hemin chloride, are allowed to react with ozone, forming a species that is tentatively assigned the structure of an oxo complex, namely, an oxo iron(IV) protoporphyrin IX radical-cation species, (PP-IX)Fe-*+(IV)=O. This species, representing the naked core of the putative active oxidant (compound 1) of heme enzymes, is characterized by its reactivity behavior in Fourier transform ion cyclotron resonance mass spectrometry, performing as an active O-atom donor. A quite distinct reactivity is displayed by an isomeric species, holding the additional oxygen on the porphyrin frame, Fe(PP-IX(O))(+). This isomer undergoes a ligand addition process, as was previously observed for Fe(PP-IX)(+).