Correlated ab initio calculations revealed that the structure of planar phosphole can be described as an intermediate between two imaginary valence isomers containing sigma(3),lambda(3)-phosphorus with two C=C double bonds (a) and sigma(3),lambda(5)-phosphorus with one C=C double bond (b). Aromatic stabilization in the case of the planar structure as calculated by proper homodesmic reactions is the largest among five-membered heteroaromatics, including thiophene. The near equal CC distances gave further support for this finding. Substituents at different positions stabilize the planar phosphole in the form of one of the two structures. pi-Acceptors, like the BH2 group, stabilize an a-type structure if placed at position 1, while a b-type structure is stabilized if the substituent is situated in position alpha. Homodesmic reactions can be written for both structures, but the two reactions predict different stabilization. If the optimized geometry is closer to the a structure, the larger stabilization was obtained in the homodesmic reaction written for the b structure and vice versa. The inversion barrier about phosphorus is significantly reduced by pi-electron acceptor substituents attached at different positions of the ring, from about 17 kcal/mol down to about 1.5 kcal/mol for some derivatives; however, none of the planar structures studied here were real minima. For the phospholes with a low inversion barrier, the structural characteristics of the nonplanar minimum, as well as the stabilizations in isodesmic reactions, are similar to those of the planar saddle points, indicating that the substituent effects noticed on the planar system can influence the characteristics and presumably the reactivity of the real molecule as well.