The heterolytic bond dissociation energies (BDEs), DH degrees(X(-)-R(+)), of X-C bonds at the P-position, relative to the pi bond in unsaturated chain compounds, can be estimated by the equation DH degrees(X(-)-R(+)) DH degrees(X(-)-C+(CH3)(m)H-3-m) - 1.4 gamma(+) + Delta V-nb + E(s)(+) kcal/mol, where R(+) represents the unsaturated chain cation (allylic, propargylic, cyanated, or others) and X is a wide variety of univalent atoms or substituents. The first three terms on the right side of the above expression are the same as those used previously for saturated compounds; tn is the degree of methyl substitution with m 1, 2, or 3 for primary, secondary, or tertiary cations; DH degrees(X(-)-C+(CH3)(m)H-3-m) are the heterolytic BDEs of model compounds; gamma(+) is the total number of framework atoms at and beyond the gamma-position, relative to the formal charge center; Delta V-nb is the steric compression relief due to the bond fission. The last term, E(s)(+), is a new parameter, the interaction energy between the empty orbital (at the formal charge site) and the pi orbital in the unsaturated cations. E(s)(+) has the value -13.4, 3.5, and 33.4 kcal mol(-1) for allylic, propargylic, and cyanated cations, respectively. The negative and positive E,+ values respectively describe the stabilization and destabilization of the given unsaturated cation, relative to its saturated analogue. Agreement between experimental and estimated values is generally very good; cases of significant disagreement are critically evaluated.