Sorption data of europium on illite were interpreted by a multisite sorption model. The model parameters were derived from the independent titration and sorption experiments. The modelling results demonstrated that the multi-site model combining the Surface Complexation (SC) and the Ion Exchange (IE) mechanisms explains the experimental data well. A significant observation was that the specific sorption, apart from the ion exchange, played an important role on the surface of illite. The acidity constants fitted from the titration data were logK(int)(+) = 5.05 and logK(int)(-) = -7.56 for the positive and negative surfaces respectively. The surface complexation constant of Eu on illite edge sites was determined to be log(Eu)K(int) = 2.13. Selectivity coefficients K-Eu(Na)C measured were not constant but dependent on background sodium concentration. Moreover, a proton exchange reaction on layer sites was postulated to interpret the observed pH dependent sorption in the low pH range.