The dissociation of water is a key elementary step in many processes. From density functional theory, we show on several transition metal surfaces (Ru, Co, Rh, Ir, Ni, Pd and Pt) that water prefers to chemisorb as a H-bonded dimer, one molecule being chemisorbed by the O atom, but the second one developing only a weak interaction with the surface. Counterintuitively, the molecule in the dimer that shows the smallest activation energy for O–H dissociation is the one interacting weakly with the surface. The H-bonded dimer provides a clear synergy for its chemisorption and assists the dissociation of the H-bond acceptor water molecule. Two different classes of O–H activation pathways are clearly identified with a linear activation energy–reaction energy relationship, of Brønstedt–Evans–Polanyi type.