Equilibrium structures have been determined for s-trans-1,3-butadiene and ethylene after adjusting the rotational constants obtained from rotational spectroscopy by vibration-rotation constants calculated from the results of quantum chemical calculations. For butadiene, the formal C=C bond length is 1.338 angstrom, and the formal C=C bond length is 1.454 angstrom. For ethylene, the C=C bond length is 1.3305 angstrom. These values appear to be good to 0.001 angstrom. It is shown for the first time that pi-electron delocalization has the structural consequences of increasing the length of the formal double bond by 0.007 angstrom and decreasing the length of the formal single bond by 0.016 angstrom. Comparisons are made with structures computed with several quantum chemical models. The MP2/cc-pVTZ results agree best with the new re structure.