Low-energy electron-energy-loss spectra of 1,3-butadiene and 1,3-cyclopentadiene deposited on a thin film of solid argon are measured at a temperature of 15 K. This new method allows us to resolve vibrational structure within the low-lying triplet bands. The vibrational frequencies observed in the first triplet band of butadiene are in agreement with theoretical predictions under the assumption of a metastable planar structure for the lowest triplet state (1 3Bu). The lowest observable vibronic band of the second triplet transition of butadiene is located at 4.63 eV. The 0–0 transition to the first triplet state of cyclopentadiene is resolved and located at 2.555 eV. Concerning the second triplet band of cyclopentadiene, we find evidence from a comparison with optical spectra that the onset lies in the energy range from 4.55 to 4.7 eV. This is the first likely observation of T2 for a cis-diene.