Raman spectra of the alkali borohydride series MBH[4] (M=Li, Na, K, Rb, Cs) have been measured as a function of temperature in the range 300–540 K. For the cubic modification of M=Na, K, Rb and Cs, the analysis of the Raman line widths suggests that the energy barrier of reorientation of the [BH4]− anions decreases as a function of cation size in the sequence Na: 12.1(5), K: 9.2(4), Rb: 8.8(3) and Cs: 8.2(4) kJ/mol. For the hexagonal high temperature modification of LiBH[4], the data suggest two energy barriers of reorientation at ~ 5 and ~ 60 kJ/mol, respectively.