A method of dissolving impure uranium tetrafluoride in a hot state in a nitric acid solution in the presence of an aluminum compound. For the purpose of obtaining a uranyl nitrate solution which can easily be separated from the solid phase formed during treatment, the dissolving is carried out in two stages at an appropriate temperature. The first stage comprises introducing quantities of nitric acid and of the aluminum compound which are insufficient to dissolve the impure uranium tetrafluoride completely, and keeping the resultant suspension agitated for a period of at least 0.5 hour. The second stage comprises introducing quantities of nitric acid and of the aluminum compound which are at least sufficient to dissolve the uranium not dissolved in the first stage, while keeping the suspension agitated.