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Where ultramafic rocks are exposed to water at temperatures < 400 °C, they inevitably undergo serpentinization reactions to form serpentine ± brucite ± talc ± magnetite (in addition to minor or trace phase like chlorite, tremolite, secondary diopside, garnet, Ni-Fe sulfides, alloys). In many circumstances, this process releases substantial amounts of hydrogen. Since the compositional controls of the primary lithology on the secondary mineralogy, fluid composition, Fe-distribution, and H₂ formation are not well established, we used thermodynamic computations to examine the equilibrium mineral assemblages, mineral compositions and the chemistry of fluids during serpentinization of 21 different ultramafic rock compositions and 10 distinct compositions of olivine between 25 °C and 400 °C at 50 MPa. Our models predict some systematic differences between serpentinization of olivine-dominated lithologies (i.e …