Mg-Ni alloys are perspective hydrogen-storage materials. In the present work, kinetics of hydrogen desorption was studied in Mg2NiH4 hydride and in hydrogenated eutectic (Mg/Mg2Ni)Hx. Time dependence of desorbed hydrogen was measured in temperature interval 552 –723 K and hydrogen diffusion coefficients were evaluated from obtained desorption curves. Results were interpreted in relation to phase composition investigated parallel by XRD. It was found that microtwinned low-temperature phase LT2 slows-down considerably the hydrogen desorption rate.