Hydrogen absorption in RMn2 Laves phases compounds lead either to RMn2Hx (where x < 4.5) interstitial hydrides under a moderate H2 pressure or to RMn2H6 complex hydrides under high H2 pressure. In the first case the structure of the hydride is a distortion of the parent compound with an increase of the cell volume whereas in the second case a new fluorite type (Fm-3m) structure is obtained. INS measurement on YMn2Hx hydrides ( 0 < x < 6) indicates a different behaviour between interstitial YMn2Hx and complex YMn2H6 hydrides, related to the difference of bonding between H and its neighbours. In addition, the INS spectrum of ErMn2D6 displays a narrow and intense peak at 213 cm-1 which is not observed in YMn2H6 and YMn2D6. In order to check the possible magnetic origin of this peak, we would like to perform complementary INS experiment on MARI.