Abstract We have performed periodic ab initio calculations to study the adsorption of hydrogen atoms on TiO 2(1 1 0) rutile surface. Two adsorption modes have been considered on slabs. Even if the presence of atomic hydrogen on the surface does not arise from the H 2 dissociation, we will refer to the H–H cleavage for the analysis. Hence, in the H mode, the heterolytic cleavage of H 2 yields to H + and H −. The proton interacts with the most basic oxygen atoms of the surface, while H − adsorbs on a metal cation which is the Lewis acidic site. In the R mode, two H + fix on top of the outer surface anions and two Ti(IV) are reduced to Ti(III). Our calculated adsorption energies values show that the R mode is the most favorable. The surface reduction is only apparent through the spin polarization and does not lead to a full electron charge transfer from the hydrogen atoms to the surface. The change of the electronic state and of the adsorption strength with the slab thickness is presented. Using NH 3 as a probe molecule to test the surface acidity, we show that the Ti reduction modifies its reactivity. Surface relaxation significantly contributes to increase the adsorption strengths; however it does not modify the qualitative conclusions.