Serpentinization of mantle peridotite generates molecular hydrogen that can be exploited by microorganisms to gain metabolic energy; however, the mechanisms that control hydrogen generation and magnetite formation during serpentinization remain poorly understood. We have examined partly to completely serpentinized peridotites recovered during the Ocean Drilling Program and fi nd a remarkable variation in the abundance of magnetite. Some completely serpentinized peridotites have as much as 6.15 wt% magnetite, whereas others are nearly magnetite free (<0.04 wt%). Using isotopic, magnetic, and thermodynamic constraints, our study reveals a simple link between the abundance of magnetite, Fe content of brucite, and serpentinization temperature. Samples with abundant magnetite have Fe-poor brucite and were formed at temperatures of 200-300 °C, whereas magnetite-poor samples are associated with Fe-rich brucite and were formed at temperatures < ~200 °C. We demonstrate that, despite the small amounts of magnetite, abundant hydrogen is generated through Fe(III) hosted in serpentine, lending support to the idea that low-to moderate-temperature serpentinization can generate abundant hydrogen within the temperature limits of life.