The geometries and energies of different isomers of model phenylpolyacetylenes and polyacetylenes have been calculated by using semiempirical molecular orbital (MNDO/PM-3) and local density functional theories. The phenyl-substituted all-trans polyenes are found to be the lowest in energy. The phenyl-substituted cis-transoid polyenes are 6.8-8.8 kcal/mol higher in energy, and the phenyl-substituted trans-cisoid polyenes are even higher in energy, 13.8-14.3 kcal/mol per two ethylene units. Second-order hyperpolarizabilities (gamma) for the different isomers of polyacetylene and phenylpolyacetylene are also calculated. A significant enhancement in gamma is predicted for cis-transoid phenylpolyacetylene as compared to the corresponding polyacetylene. Geometry differences are expected to be an important component of this enhancement.