Employing femtosecond IR pump−probe and 2D spectroscopy, we measure the vibrational dynamics of LiBH4 and several of its deuterium isotopomers. We find that the vibrational lifetime of various BH and BD stretching modes uniformly is ≈1.5 ps for all BH4−xDx− units (0 ≤ x ≤ 4). Subsequently, vibrational energy cascades down through BH and BD bending, facilitated by a strong Fermi resonance, into external (librational and translational) modes with a 3 ps time constant. Final thermalization of the energy is completed in about 100 ps. The vibrational spectra are purely homogeneously broadened indicating low inhomogeneity due to static disorder. We furthermore measured the anharmonic constants of various modes, which sets the benchmark for future ab initio calculations, and completed the first FTIR assignment of the stretching vibrations for the five BH4−xDx− units.