The kinetics of oxidation of uranium(IV) monofluoride complex by nitrous acid in nitric acid solution have been studied. The experiments were carried out at constant ionic strength of 2M (HNO3 and NaNO3) and temperature in the range of 18–47 °C. The rate of reaction was determined spectrophotometrically at a wavelength of 621 nm, at which the molar extinction coefficients of UF3+ and UF
are the same. It was shown that reaction orders for [HNO2] and [HNO3] are equal to 0.12 and 0.39, respectively. The values of activation parameters H and S are determined to be 83 kJ mol–1 and 75 J (mol·K)–1, respectively. The rate order of the reaction studied has a weak direct dependence on [H+] in contrary to the strong and reverse dependence in the absence of fluoride ions. In conclusion, fluoride ions may strongly stabilize the U(IV) in nitric acid solutions.