1. In solutions containing hydrochloric acid in sufficiently high concentration, uranyl ion is almost quantitatively reduced to uranous ion in the mercury reductor. 2. The simultaneous presence of both hydronium ion and chloride ion in high concentration is necessary. This may be explained from the E₀ potentials of the uranous-uranyl and the mercury-mercurous systems. 3. By the use of a small correction factor this reduction may be applied to the volumetric determination of uranium. The possible interference of uranium should be taken into account when using the mercury reductor for the determination of certain other elements.