Carbon supported Pd Ni nanoalloys with well controlled composition and particle size distribution were synthesized by incipient wetness impregnation method. The Pd-rich nanoalloys (10 and 20 Ni at.%) form a hydride phase at atmospheric pressure and ambient temperature whereas, no hydride formation is observed for higher Ni content (30 at%), as determined by Pressure-Composition-Isotherms and in situ X-ray diffraction. Significant thermodynamic changes for hydride forming Pd Ni nanoalloys are noticed as compared to pure Pd, in agreement with the bulk counterparts: the enthalpy of the hydride formation of Pd Ni nanoalloys decreases by increasing the Ni content. These thermodynamic changes are not related to downsizing of particles but rather to an alloying effect. The solely effects of particle downsizing are the sloping and the shortening of the plateau observed in the PCI curves. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.