Significance Gold is the noblest metal. However, when the size decreases to nanoscale and is supported on reducible oxides, the gold nanoparticle shows exceptionally high catalytic performance even at low temperatures. Here, through state-of-the-art in situ aberration-corrected environmental transmission electron microscopy and ab initio molecular-dynamic simulations, we discovered that, upon exposing to reactant gas (carbon monoxide and oxygen), ultrasmall gold clusters on ceria show a size-dependent order-to-disorder transformation with generation of dynamic low-coordinated atoms, which presumably can effectively boost the oxidation reaction of carbon monoxide. The findings provide much-needed insights on the origin of size-dependent catalytic properties of supported gold and demonstrate a size effect in absorbent–particle interactions that may widely exist and play an essential role in heterogeneous catalysts.