A series of new osmium(II) porphyrin oligomers having tetraarylporphyrins with mixed 4-pyridyl (Py) and phenyl (P) groups, H(2)Py(n)P(4-n)P, as an axial or a bridging ligand, Os(OEP)(CO)(H(2)PyP(3)P), 1, cis-[Os(OEP)(CO)](2)(H(2)Py(2)P(2)P), 2, trans-[Os(OEP)(CO)](2)(H(2)Py(2)P(2)P), 3, [Os(OEP)(CO)](3)(H(2)Py(3)PP), 4, and [Os(OEP)(CO)](4)(H(2)TPyP), 5, were synthesized and characterized by spectroscopic methods. The oligomerizations were confirmed by H-1 NMR. The construction of the oligomers enabled to observe directly the tautomerization of internal N-H protons in the axial porphyrins at low temperatures. The activation energy of the tautomerism : E(a) = 40.5 kJ mol(-1), and the exchanging rate constant, k = 7200 +/- 100 s(-1) at 298 K, were comparable to the values previously reported for various porphyrin monomers. The absorption spectra of these oligomers are essentially superpositions of the spectra of the constituent monomeric porphyrins. The cyclic voltammograms of all the oligomers showed the redox waves of parent porphyrins and the osmium ions, each current reflecting the number of constituents. The peak separations of these Os(II/III) redox waves are nearly the same among the five oligomers. The redox and electronic spectral measurements of the oligomers indicated that there are no distinct interactions between the macrocycles in the ground state electrochemically and visible spectrally.