Valence considerations suggest the plane symmetrical form (point group D2h) for an ethylene molecule. The observed spectra show that there are no bands that occur in the Raman as well as the infrared spectrum of an ethylene gas, proving that the molecule has D2hsymmetry where the molecule has a center of symmetry. From valence theory one would expect for an allene molecule that the C = C = C chain is linear but that the two CH2groups lie in planes at right angles to each other and passing through the C = C = C axis, that is, that the molecule belongs to point groups D2d. The obtained Raman and infrared data of an allene can be interpreted according to D2dsymmetry.