Superposition of significant electron structures is used to estimate the mesomerism stabilization of polycyclic benzenoid systems by consideration of the number ofKekule structures and of benzenoid interactions between them. General formulae and stability rules are given for many classes of cata and pericondensed molecules. The decoupling of large benzenoid systems into isolated subsystems is described. Three distinct types of polymeric benzenoid systems exist: unstable systems with vanishing mesomerism energy per monomeric umit, and stable systems with nonvanishing mesomerism energy, which are or are not decoupled into isolated subsystems.