The titration of a (2x2)p2mg-O/Rh(110) surface by hydrogen and the subsequent deconstruction of the nonequilibrium (1x2) reconstructed surface has been studied by scanning tunneling microscopy (STM) in a temperature range between 380 and 405 K. The titration reaction is seen to occur via reaction fronts, which start at steps, are elongated in the [1 (1) over bar0] direction and leave behind a surface covered by diffusing oxygen atoms. Two deconstruction mechanisms are active: A long-range row pairing of adjacent (1x2) rows, which propagates over several hundreds of Angstrom in the [1 (1) over bar0] direction, and a local hole formation in which Rh atoms detach from the ends of broken [1 (1) over bar0] rows, diffuse over up to 100 Angstrom and condense into (1x1) islands. Both mechanisms lead to the formation of correlated up-down islands; at step edges, the second one leads to a step retraction. The deconstruction is strongly defect mediated and the STM results indicate that the (1x2) reconstructed surface is most probably metastable.