We propose to study two aspects of the hydrogen dynamics in complex hydrides:First: The structural transitions in borohydrides are believed to be connected with an increase in rotational freedom of the BH4 groups in the high temperature phase, enhancing the diffusion. On the IRIS spectrometer at ISIS, we propose to explore rotational diffusion of the BH4 moiety in the low temperature phases of NaBH4 and KBH4 (below 200K and below 60K, respectively). Second, We propose to measure elastic scans (for NaBH4 and LiBH4), covering the whole temperature range, to get an overview over the thermal behaviour of the samples. These scans will then allow defining the temperatures for subsequent inelastic spectra from which plan to deduce the activation energies. Access to the Q-dependence of spectral linewidths and intensities may allow us to get information on translational motions.