The crystal structure of diacetato(N-methyl-meso-tetraphenylporphyrinto)thallium(III), Tl(N-Me-tpp)(OAc)(2) (1), was established, and the coordination sphere around the Tl3+ ion is described as an eight-coordinate square-based antiprism in which two cis chelating bidentate OAc- groups occupy two apical sites. The plane of the three pyrrole nitrogen atoms (i.e., N(1), N(3), N(4)) strongly bonded to Tl3+ is adopted as a reference plane 3N. The pyrrole N(2) ring bearing the methyl group (i.e., C(45)H-3) is the most deviated one from the 3N plane, making a dihedral angle of 21.4 degrees, whereas smaller angles of 9.1 degrees, 7.1 degrees, and 0.9 degrees occur with pyrroles N(1), N(3), and N(4), respectively. Because of its larger size, the thallium(III) ion Tl3+ is considerably out of the 3N plane; its displacement of 1.17 Angstrom is in the same direction as that of the two apical OAc- ligands. The intermolecular acetate exchange process for 1 in THF-ns solvent is examined through H-1 NMR temperature-dependent measurements. In the slow-exchange region, the methyl and carbonyl carbons of the OAc- groups in 1 are separately located at delta 18.6 [(3)J(Tl-C-13) = 405 Hz] and 170.8 [(2)J(Tl-C-13) = 334 Hz] at -80 degrees C, respectively.