The ability to measure 238U/235U to high precision presents an important new opportunity to study the fate and transport of uranium in the environment. The ratio of 238U/235U was determined by multicollector–inductively coupled plasma–mass spectrometer in six uranium ore samples representing two different classes of deposits. Significant offsets in 238U/235U are observed between uranium ores precipitated from groundwaters at low temperature versus hydrothermal deposits precipitated at high temperatures, reinforcing an observation made previously but lacking the needed precision. Specifically, tabular sandstone-type uranium deposits were found to be depleted in 235U, with a total offset between low-temperature deposits and higher temperature deposits of ≈1.0‰. We attribute this offset to reflect a temperature-dependent fractionation related to the nuclear field shift effect during chemical reduction of uranium in ambient temperature groundwaters.