The availability of diphenyl-23-oxa-, -thia-, and -selena-21-carbaporphyrins has enabled the reactivity of these systems to be investigated and contrasted. All three heterocarbaporphyrins reacted with palladium(II) acetate in refluxing chloroform acetonitrile to give organometallic palladium(II) derivatives in good yields. These structures are stable and give UV-vis spectra that show increasing broadening and bathochromic shifts as the size of the heteroatom increases. Nickel(II) acetate in refluxing N,N-dimethylformamide reacted with the oxa- and thiacarbaporphyrins under nitrogen to give the corresponding nickel(II) complexes, but the selenacarbaporphyrin did not metalate under these conditions. The NMR spectra for all of the metal complexes showed that they possess strong diamagnetic ring currents, although the palladium complexes gave larger downfield shifts that were slightly diminished when larger heteroatoms were present in the porphyrinoid cavity. Reactions of the oxacarbaporphyrin with nickel(II) acetate in the presence of air led to a unique oxidation reaction that afforded a weakly diatropic 21-oxycarbaporphyrin, and low yields of a related product were also obtained from the thiacarbaporphyrin.