Complete scaled ab initio force fields and frequencies have been calculated for the all-trans and all-cis isomers of 1,3,5,7-octatetraene. Several peaks in the infrared and Raman spectra have been reassigned, and a medium intensity IR peak at 1580 cm-1 is predicted. The agreement between the experimental and theoretical C-C stretching frequencies is very good. We believe that the force constants are superior to previous theoretical ones. A detailed comparison with the results from an extended Pariser-Parr-Pople configuration interaction theory is made. The effect of cis-trans isomerization is discussed. The perdeuterated frequencies are also presented in anticipation of future experimental measurements. © 1989 American Chemical Society.