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The valence effective Hamiltonian technique is applied to a series of polymers to compute ionization potentials, bandwidths, and band gaps. The polymers considered represent systems of interest to the conducting polymers area and include various derivatives of polyacetylene and polyphenylene, polydiacetylene, polyacene, polybenzyl, and polyyne. The theoretical results for relative ionization potentials are in excellent agreement with available experimental estimates, as well as with the observed behavior of the electrical conductivity of these systems on exposure to weak (I₂) versus strong (AsF₅) electron acceptors. The bandwidths of the highest occupied band show a qualitative correlation to the conductivities achieved with acceptor doping. Band gaps for the planar systems considered are also in good agreement with experiment.