The far i.r. (FIR) spectra of symmetric aliphatic ketones C13 through C27 are reported for temperatures 85 K and ≤ 15 K over the frequency range 5-300 cm-1. The structure of the skeletal internal and lattice modes and their relationship to the purely intra-molecular vibrational modes of isolated molecules are determined and the problems of assigning the observed FIR absorption lines are discussed. © 1979.