The adsorption of formic acid on a ZnO(1 0 -1 0) surface has been investigated with low temperature scanning tunneling microscopy (LT-STM) in combination with density-functional theory (DFT) calculations. Through high resolution STM images and in-situ exposure experiments we clearly identify that the formic acid dissociates upon adsorption even at liquid nitrogen temperature. DFT calculations reveal a preferential bidentate configuration of the formate species with the molecular plane orientating along the [1 -2 1 0] direction, while the yielded surface hydroxyl has two kinds of anchoring sites with similar energy. At elevated temperature, the adsorbed molecules were found to accumulate into short chains consisting exclusively of formate and hydroxyl and orienting along the [0 0 0 1] direction, which finally assemble into a (2 x 2) superstructure at half monolayer coverage. Careful examination of the defect structure of the saturation coverage film revealed the existence of monodentate adsorption states of formic acid but with very small probability. These results have clarified the inconsistent propositions from different spectroscopic studies. Moreover, the preferential ordering of the formate species may bring fresh insights into the understandings of the related surface reactions through formate intermediates. (c) 2020 Elsevier Inc. All rights reserved.