In this work, the effort is directed to assess the best source term conceptual model that represents Cs release from 14 different cementitious waste matrices. These matrices were divided into three main categories; the first includes simulated Intermediate Level Waste (ILW) immobilized in different cementitious media, i.e. Ordinary Portland Cement (OPC), Blast Furnace Slag (BFS), and a mixture of both of them. The second category includes cementitious OPC-ILW matrices with different inorganic resins additives. The last category contains OPC-ILW-polymer immobilized matrices with different organic exhausted resins. Cs releases from the studied matrices have been investigated according to IAEA recommended leach test. The cumulative leach fraction of Cs was found to be less than 5% for all the studied matrices, this implies the applicability of IAEA recommended method to estimate the diffusion coefficient. The experimental data were linearly and nonlinearly regressed to different developed source term conceptual models. The regression results indicated that Cs releases are best described by first-order reaction/diffusion model (FRDM). FRDM regression results were compared to that of the IAEA recommended method to estimate the diffusion coefficients by performing statistical and uncertainty analysis. The results of the comparison revealed that using FRDM gives more realistic prediction for the cumulative leach fraction for the first and second waste categories, where IAEA recommended method produces more realistic estimation for the third waste category. (C) 2011 Elsevier B.V. All rights reserved.