Abstract In this paper there is discussed the interaction of two conjugated systems across a conj ugated single bond. It is shown that the conjugation energy across the bond, and the mobile order of the bond, are both positive, and that both are closely related to the self-polarizabilities of the atoms forming the bond. This relation forms the basis for a definition of the conjugating power of a position as β times its self-polarizability. The conjugating power of various hydrocarbon residues is shown to increase in the order: phenyl, β-naphthyl, 2-butadienyl, α-naphthyl, vinyl, 1-butadienyl.