Adsorption of formic acid (HCOOH) on ZnO(10 (1) over bar 0) surfaces has been investigated by means of quantum-chemical ab initio periodic Hartree-Fock calculations. One 'bridging' structure, characterized by a formate anion symmetrically bonded via its two oxygens to two surface Zn atoms and the acidic proton bound to a surface oxygen, and two 'unidantate' structures, characterized by one Zn-surf-O-ads bond and the acidic proton bound to a surface oxygen, were investigated. It was found that a tilted bridging geometry represents the most stable adsorption mode, with an adsorption energy of 52.4 kcal/mol. A bridging structure tilted by 15 degrees is energetically favourable compared to an upright bridging structure by 6.9 kcal/mol, which makes the bridging adsorption mode 6.3 kcal/mol more favourable than the most stable unidentate adsorption mode. The dependence of the adsorption energy on the surface coverage was also investigated. In addition, the structures of the ZnO bulk and of the clean surface have been calculated