Significance The catalytic activity of metal particles is highly size-dependent in the subnanometer regime, which makes understanding how and why particle sizes change in reactive atmospheres particularly important. Here, we show that carbon monoxide plays a dual role in the coarsening of otherwise highly stable Pt atoms on an Fe 3 O 4 (001) support: CO adsorption weakens the adatom–support interaction inducing mobility, and stabilizes the Pt dimer against decay into two adatoms. Our results illustrate how molecules modify the clustering dynamics on surfaces, provide much-needed insight into how deactivation and redispersion can occur in single-atom catalyst systems, and demonstrate an approach to prepare size-distinguished clusters for studies of the size effect.