The discrepancy between the rao and rz structures of acrolein reported in a previous study has been removed by taking into account small isotopic differences in the distances among the carbon and oxygen atoms. The effect of uncertainties in the isotopic differences on the determination of the average structure is examined, and a general method for deriving the best possible structure from electron-diffraction and spectroscopic data is suggested. The rg bond distances in butadiene, acrolein, and glyoxal are compared with one another. © 1969.