The molecular structure and conformational energies of methyl acrylate, methyl methacrylate, vinyl acetate, methyl propionate, methyl 2-methylpropionate and styrene have been investigated using PCILO (perturbative configuration interaction with localized orbitals) and AM1 (Austin model 1) semiempirical methods with total optimization of geometry. The minimum-energy conformations obtained with both methods have been compared. The heats of formation calculated with the AM1 method are in excellent agreement with experimental values. The rotational barriers were also calculated and there are similarities in shape between the various compounds that do not involve an ester oxygen.