A systematic study of the adsorption and dissociation of water on transition and noble metal dimers (Fe2, Ru2, Os 2, Co2, Rh2, Ir2, Ni2, Pd2, Pt2, Cu2, Ag2, Au2) is presented. Spin-unrestricted density functional theory simulations indicate that the dissociation (lysis) of water on these clusters may be thermally driven even in the absence of other electrocatalytically and photocatalytically driven processes. Two reaction pathways are found with turnover frequencies for water dissociation given by the following series: Co2 > Ir2 > Fe2 > Rh2 > Ni2 > Pt2 > Ru2 > Os2 > Cu2 > Au2 > Pd2 > Ag2 at standard ambient temperature and pressure. Linear free energy relationships are presented that can predict the dissociation barrier of water on transition and noble metals as a function of free energy change for dissociation. © 2009 American Chemical Society.