Crystal facets, vertices and edges govern the energy landscape of metal surfaces and thus the chemical interactions on the surface(1,2). The facile absorption and desorption of hydrogen at a palladium surface provides a useful platform for defining how metal-solute interactions impact properties relevant to energy storage, catalysis and sensing(3-5). Recent advances in in operando and in situ techniques have enabled the phase transitions of single palladium nanocrystals to be temporally and spatially tracked during hydrogen absorption(6-11). We demonstrate herein that in situ X-ray diffraction can be used to track both hydrogen absorption and desorption in palladium nanocrystals. This ensemble measurement enabled us to delineate distinctive absorption and desorption mechanisms for nanocrystals containing exclusively (111) or (100) facets. We show that the rate of hydrogen absorption is higher for those nanocrystals containing a higher number of vertices, consistent with hydrogen absorption occurring quickly after beta-phase nucleation at lattice-strained vertices(9,10). Tracking hydrogen desorption revealed initial desorption rates to be nearly tenfold faster for samples with (100) facets, presumably due to the faster recombination of surface hydrogen atoms. These results inspired us to make nanocrystals with a high number of vertices and (100) facets, which were found to accommodate fast hydrogen uptake and release.