Binding mechanisms and leaching characteristics of cesium from different cement-bentonite immobilization matrices were investigated. The effect of Sr presence as a competitive contaminant in the matrices was studied by investigating the binding and leaching mechanisms in binary contaminant matrices that contains both Cs and Sr solutions. Binding investigations aimed to trace Cs binding mechanisms by calculating the distribution of contaminant and major structural elements aqueous complexes in the mixing solution and interstitial pore fluid before the hydration reaction take place. The hydration phases development within 28 day curing period was studied and their effects on the speciation of contaminant and major structural elements were investigated. The leaching characteristics were studied by analyzing the experimental results of the long term leaching test. The results indicated that the percentage distribution of aqueous Cs and Sr complexes in their solution are independent on the presence of each other. Cs was found as free ions in the interstitial pore fluid for all the studied cases. The interstitial pore fluid pH was found to be dependent on the presence of bentonite and sodium was found as dissolved Na+1 ions, this might form evidence that Cs+1 might substitute Na+1 in bentonite structure. The intermolecular channels in Ettringite and CSH structures were found to contribute to the physical entrapment of Cs and Sr ions within the solidified matrices. The leaching characteristics were resulted from a combination of first-order, diffusion, dissolution and instantaneous release of contaminants mechanisms. Dissolution and instantaneous release mechanisms were found to have very low contribution to the leaching process. (C) 2014 Elsevier B.V. All rights reserved.