Six new uranium phosphites, phosphates, and mixed phosphate-phosphite compounds were hydrothermally synthesized, with an additional uranyl phosphite synthesized at room temperature. These compounds can contain U-VI or U-IV, and two are mixed-valent U-VI/U-IV compounds. There appears to be a strong correlation between the starting pH and reaction duration and the products that form. In general, phosphites are more likely to form at shorter reaction times, while phosphates form at extended reaction times. Additionally, reduction of uranium from U-VI to U-IV happens much more readily at lower pH and can be slowed with an increase in the initial pH of the reaction mixture. Here we explore the in situ hydrothermal redox reactions of uranyl nitrate with phosphorous acid and alkali-metal carbonates. The resulting products reveal the evolution of compounds formed as these hydrothermal redox reactions proceed forward with time.