By means of scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we characterize at the single-atom level the mechanism of the water formation reaction on the (10 x 2)-O/Rh(110) surface, a prototype of a one-dimensional (11 D) oxide where the lattice expansion and the segmentation of the surface play a fundamental role. When the reaction is imaged in the 238-263 K temperature range (35 s/image acquisition time), a peculiar comblike propagation mechanism for the reaction front is found. Fast STM measurements (33 ms/image) prove that this mechanism holds also at room temperature, being therefore an intrinsic characteristic of the reaction on the 1 D oxide. DFT calculations explain the observed behavior as due to the interplay between the lattice expansion in the initial surface and its relaxation during the reaction that leads to varying configurations for the reactants. At low temperatures, the reaction produces, in its final stages, a low-coverage, ordered patterning of the surface with residual oxygen. The pattern formation is related to the segmentation of the oxide phase.