A carrier method was used to measure the dissolution equilibrium of iron vapour over liquid U-Fe alloys from 1500 to 1600 °C, and then the activities as well as activity coefficients of iron in liquid U-Fe alloys were calculated from the results for iron. It is found that iron shows strong negative deviation from Raoult’s law, and the relationship between the standard free energy of dissolution reaction Fe(g)=[Fe]Y and temperature is DG0=–448682+125.4T. At 1500 and 1600 °C, the activity coefficients g of iron at infinite dilution are 0.344 and 0.485, respectively.