Experimental studies have shown that temperature, pressure, sulfur fugacity (fS2), and oxygen fugacity (fO2) influence the Fe content of sphalerite. We present compositional in situ data on sphalerite from submarine volcanic-hosted massive sulfide (VHMS) ores of hydrothermal vents from different plate tectonic settings and with variable host-rock compositions. Sphalerite from sediment-hosted vents has systematically higher S contents and Fe/Zn ratios than those of the sediment-starved vents, reflecting an influence of fS2 and fO2 on Fe partitioning between fluid and sphalerite. The Fe/Zn ratios of sphalerite from sediment-starved vent systems apparently increase systematically with the fluid temperatures of the corresponding vents. We conclude that the composition of sphalerite can be used to (1) distinguish between sediment-hosted and sediment-starved hydrothermal processes, and (2) estimate minimum fluid temperatures of sphalerite precipitation from inactive sediment-starved hydrothermal vent sites and fossil VHMS deposits.