trans-Dichloro-, trans-dibromo-, and trans-diiodoosmium(IV) tetraarylporphyrins were obtained by extremely facile synthetic routes directly from the reactions of the corresponding (carbonyl)osmium(II) complexes with CCl4, CBr4, and CI4, respectively. At short reaction times, appreciable amounts of intermediates-one for each reaction-were observed by spectroscopic investigations. These intermediates were shown to be (carbonyl)(halo)(porphyrinato)osmium(III) complexes by independent preparation of an authentic (carbonyl)(bromo)(porphyrinato)osmium(III) complex, which was identical to the reaction intermediate in the reaction of CBr4 and very similar to those of the other reactions. This provided strong evidence for the reaction mechanism, two stepwise one-electron oxidations of the metal ion. The relatively strong binding of carbon monoxide to osmium(III) is proposed to be an important factor in avoiding dimerization of the reaction intermediates.