The conjugative ability of the lambda(5)-P=C bond has been compared to its lambda(3)-P=C counterpart at the MP2/6-31G*//MP2/6-31G* level of theory, using isodesmic reaction energies. Investigating heterobutadienes, it has been observed that compounds containing a lambda(3)-P=C bond show similar delocalization energy as those with C=C units. As for lambda(5)-phosphabutadienes, however, stabilization is achieved only in the case of C substitution. This behavior has been rationalized by perturbation theory arguments with the conclusion that while for the lambda(3)-P=C and C=C bonds a two-way interaction, for the lambda(5)-P=C system a one-way interaction is operational, Comparing cyclic systems containing lambda(3)- and lambda(5)-P=C bonds, similar structures (bond lengths) and stabilizations (as concluded from bond separation and homodesmic reaction energies) were obtained for six-membered rings including benzene and other rings containing one and three (symmetrically arranged) phosphorus atoms. The four-membered rings (1 lambda(3),3 lambda(3)- and 1 lambda(5),3 lambda(5)-diphosphacyclobutadienes), however, show completely different behavior, since the lambda(5)-P derivative does not show destabilization upon ring formation and has equal bond lengths, while lambda(3)-P rings are clearly antiaromatic. Rationalization has been given in terms of the differences in the one-way and two-way conjugative interactions.