SummaryThe absorption spectra of 11 solid-state EDA-complexes of unsaturated hydrocarbon donors with TCNE as acceptor have been studied in the FIR. region (40-400 cm-'). The dominant feature in the spectra is the appearance of a new intense absorption in the region 80-100 cm-' which is assigned to the intra charge-transfer lattice mode. The (idealized) force constant for this lattice mode is found to be roughly constant for most of the EDA-complexes and is thus believed to be a measure of the total interaction of donor and acceptor within the crystal site. Force constants differing in magnitude from those of the majority of complexes are traced back to a particular structure of the donor component and/or the EDA-crystal as a whole. A recently proposed reassignment of the Raman active fundamentals of pure TCNE is questioned in view of the present findings.