The local structure and the thermal stability of small and well-dispersed RhHx nanoparticles (average size of 1.4 nm) were studied by in situ X-ray Absorption Spectroscopy. The RhHx nanoparticles are stable at room temperature and undergo a structural transition from hydride (fcc) to metal phase (fcc) with a shrinking of the lattice volume due to the desorption of hydrogen. This phase transition occurs in the temperature range of 150-180 degrees C, in good agreement with the results from thermo-desorption spectroscopy. Above 180 degrees C, the desorbed nanoparticles undertake important coalescence. In situ transmission electron microscopy performed up to 300 degrees C proves that this process cannot be only thermal, thus it may be ascribed to a X-ray beam effect. (C) 2018 Elsevier Inc. All rights reserved.