IR and Raman investigations have been carried out on purified Li@C-60 and Li@C-70 produced by low-energy ion implantation. The structural interpretations, based on theoretical considerations and comparison of the obtained vibrational spectra with those of different known fullerene oligomers, have shown that neither of the chromatographically isolated endohedral species (Li@C-60(70)) correspond to monomeric nonderivatized molecules. We ascribe the two different fractions of Li@C-60 to a double-bonded dimer and a trimer, presumably with the shape of closed triangle. The infrared and Raman spectra of the Li@C-70 compound are similar to those Of (C-70)(2), and we therefore propose for this species the structure of a double-bonded dimer (Li@C-70)(2). The IR spectra of the Li@C-60 fractions prepared with Li-6 and Li-7 isotopes are identical except for the position of broad bands at about 450-600 cm(-1), originating from vibrational-rotational movement of the Li+ cation inside the carbon cage.