The tgtg’ conformation of poly(vinylidene fluoride) (PVDF) is similar to 0.5-0.7 kcal mol(-1) monomer(-1) more stable than the all-trails conformation as calculated by molecular mechanics. For vinylidene fluoride-trifluoroethylene copolymer compositions < similar to 90 mol% vinylidene fluoride, the all-trans conformation is more stable. For PVDF. crystal packing calculations support the distorted tgtg’ conformation predicted by X-ray crystallography.