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 find 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.