[Display omitted] • The dissociative adsorption of H 2 on ZnO(10-10) is raised from 50 K to 200 K by preadsorbed formic acid. • In-situ STM experiments directly observe H-chains grow selectively aside the type-II formic acid. • DFT reveals the accumulated charges around the preadsorbate account for the promoted H 2 adsorption. Activation of hydrogen on oxide surface is of both practical and fundamental importance for broad fields of syngas-based chemical industries as well as clean energy utilizations. Previously we found ZnO can catalyze the heterolysis of H 2 , which only occurs at low temperatures (<50 K) due to the quick kinetics of the reversed reaction. Here in this work, we demonstrate that the heterolytic adsorption of H 2 can be significantly enhanced by pre-adsorbed formic acid molecules. The formic acid molecule can readily dissociate on the ZnO(10−10) surface and form strongly-bound formate and hydroxyl species, which induce charge accumulation at the nearest-neighbored Zn ions and initiate the heterolysis of H 2 molecules as well as the growth of H-chain structure along the [0001] direction on the ZnO surface. With such stabilization effect of formic acid, the dissociative adsorption of H 2 can be significantly raised up to 200 K under UHV conditions. These results strongly emphasize the important impact of co-adsorbates on the surface reaction process. Particularly, our findings may shed new light on the atomistic mechanisms for a variety of H 2 -involved reactions on oxide catalysts. [ABSTRACT FROM AUTHOR]