The synthesis, structures, optical and redox properties, and reactivity of phosphaporphyrins are reported. The 21-phosphaporphyrin (P,N(3)-porphyrin) and 23-phospha-21-thiaporphyrin (P,S,N(2)-porphyrin) were prepared via acid-promoted dehydrative condensation between a phosphatripyrrane and the corresponding 2,5-bis[hydroxy(phenyl)methyl]heteroles followed by 2,3-dichloro-5,6-dicyanobenzoquinone oxidation. Experimental (NMR, UV-vis, and X-ray analyses) and theoretical (DFT calculations) results suggest that the 18pi aromaticity inherent in regular N(4)-porphyrins was maintained in these phosphaporphyrins. X-ray crystallography revealed a slightly distorted 18pi aromatic ring for the P,N(3)-porphyrin with the phosphole and three pyrrole rings tilted from the 24-atoms mean plane by 9.6 degrees and 3.8-15.4 degrees, respectively. DFT calculations on model compounds showed that the P,X,N(2)-porphyrins (X = N, S) possess considerably small HOMO-LUMO gaps as compared with N(4)- and S,N(3)-porphyrins, which is reflected in the red-shifted absorptions, low oxidation potentials, and high reduction potentials of the phosphaporphyrins. The P-oxygenation of the P,X,N(2)-porphyrins with H(2)O(2) has been found to lead to the formation of different types of products. The 18pi P,N(3)-porphyrin was transformed into the 22pi aromatic P(O),N(3)-porphyrin accompanied by the pi extension at the peripheral C(3) bridge, whereas the 18pi P,S,N(2)-porphyrin was converted to the isophlorin-type 20pi antiaromatic P(O),S,N(2)-porphyrin. In both of the reactions, simple P-oxygenated 18pi P(O),X,N(2)-porphyrins were formed as the initial products, which were subsequently transformed into the 22pi or 20pi porphyrins. The two reaction courses from 18pi to 20pi/22pi are apparently determined by the combination of the core heteroatoms (i.e., P,N(3) or P,S,N(2)) and the structure of the peripherally fused carbocycles. The present results demonstrate that the incorporation of a phosphorus atom into the core is not only a highly promising way to modify the fundamental properties of the porphyrin 18pi system but also a reliable tool to stabilize uncommon 22pi and 20pi systems through the chemical modifications at the core phosphorus atom.