The effect of fluoride ion upon uranium(IV) solutions containing ethylenediamine tetraacetic acid is explained in terms of the participation of the UF 3+ ion in co-ordination. The stability constant of this ion with ethylenediamine tetraacetic acid has been measured and its value used to estimate thermodynamic quantities associated with the formation of the complex fluoride. The general co-ordinating tendency of UF 3+ ion is discussed and compared with other systems of a similar nature.