We monitor the hydrogenation of PTFE/Pd-capped MgyNi1-yHx gradient thin films using the change in optical transmission as a function of time, temperature and hydrogen pressure, to study the relation between kinetics and thermodynamics of hydrogenation of this multiphase hydride system. The interplay between kinetics and thermodynamics is used to extrapolate the hydrogenation equilibrium pressures via the H-absorption rate. Pressure-optical-transmission-isotherms determined independently at different temperatures provide a cross-check for the equilibrium pressure and the enthalpy of H-absorption. We find that the hydrogenation reaction is destabilized with respect to the Mg2Ni -> Mg2NiH4 reaction for Mg fractions close to the Mg2Ni-Mg eutectic point. From a comparison with calculated enthalpies obtained from density functional theory, we conclude that the experimentally observed destabilization originates from the well-mixed microstructure around the eutectic point. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.