Abstract: The stability constants of complexes of Eu,Gd,Tb,Yb with fulvic acid from weathered coal are determined using Schubert's ion exchange method and radioactive tracers. The range of stability constants is 108 - 1014,and the 1:1 and 1 : 2 binding sites complexes are the predominant species.Calculation methods based on Ardakani-Stevenson's ion exchange equilibrium method are developed for determining stability constants of metal with humic and fulvic acids. The methods are verified using Tb(Ⅲ)-fulvic acid from weathered coal. The stability constants range from 1011.93 - 1011.15 at pH 4. 7 and ionic strength 0. 1mol/kg.