In this work, phase compositions and chemical valence states on the surface and subsurface of Mg67Ni33-xYx (x=0, 1, 3, 6) ribbons during thermal activation have been investigated by X-ray photoelectron spectroscopy (XPS). The results indicate that the surface contaminants of melt-spun ribbons are mainly MgO, NiO, Y2O3 and organics. The oxides/hydroxides of Mg67Ni33-xYx (x=0, 1, 3, 6) melt spun ribbons are removed from the surface during thermal activation. Surface chemical valence firstly transforms from oxidized state to the metallic one during thermal activation, which accounts for hydrogenation of Mg67Ni33-xYx. melt-spun ribbons. Hydrogen absorption capacities of Mg67Ni33-xYx. (x=0, 1, 3, 6) melt-spun ribbons are enhanced with the increase of cycle numbers during thermal activation. Hydrogenation thermodynamics of activated Mg67Ni33-xYx. (x=0, 1, 3, 6) melt-spun ribbons have been also compared and correlated with the surface valence transformation. The obtained enthalpy of hydride formation is -55.5, -50.5, -46.9 and -48.6 kJ/mol for Mg67Ni33-xYx melt-spun ribbons with x=0, 1, 3 and 6, respectively. (C) 2016 Elsevier B.V. All rights reserved.