Uranyl formate (UO<sub>2</sub>(CHO<sub>2</sub>)<sub>2</sub>÷H<sub>2</sub>O) is a salt that exists as a fine yellow free-flowing powder occasionally used in transmission electron microscopy.
It is used as a negative stain in transmission electron microscopy (TEM) because it exhibits a finer grain structure than uranyl acetate. However, uranyl formate does not easily dissolve, and once in solution, it has a limited lifespan as a stain. Typical aqueous solution concentrations are 0.5% or 1%. It is highly sensitive to light, especially ultraviolet light, and will precipitate upon exposure.