This interdisciplinary work combines the use of shape- and size-defined Pd nanocrystals (cubes of 10 and 18 nm, and octahedra of 37 nm) with in situ techniques and DFT calculations to unravel the dynamic phenomena with respect to Pd reconstruction taking place during acetylene hydrogenation. Notably, it was found that the reacting Pd surface evolved at a different pace depending on the shape of the Pd nanocrystals, due to their specific propensity to form carbides under reaction conditions. Indeed, Pd cubes (Pd(100)) reacted with acetylene to form a PdC0.13 phase at a rate roughly 6-fold higher than that of octahedra (Pd(111)), resulting in nanocrystals with different degrees of carburization. DFT calculations revealed changes in the electronic and geometric properties of the Pd nanocrystals imposed by the progressive addition of carbon in its lattice.