A rapid method for determining triuranium octoxide (U₃O₈) in geologic, biologic, and water samples by fluorometry is described. Release of total U₃O₈ from geologic samples was accomplished by fusing samples with lithium tetraborate at 1040°C (1904°F) and subsequently dissolving the molten sample in 1.6N nitric acid. U₃O₈ in biologic samples and in association with organic substances in water was liberated by oxidation with concentrated nitric acid. U₃O₈ in 1.6N nitric acid was complexed with trioctylphosphine oxide and extracted into cyclohexane. The recovery of 0.1-μg U₃O₈ in the presence of 100 to 500 μg each of potential interfering elements ranged from 99 to 105%. Results on reference samples corresponded well to the reported values. The new method proved to be more reproducible and efficient than the method that uses ethyl acetate extraction from a saturated aluminum nitrate solution for determining total U₃O₈ in geologic samples.