10 pages.-- Final full-text version of the paper available at: http://dx.doi.org/10.1016/j.jhazmat.2006.04.059.The efficiency of innovative matrices for immobilizing cesium is presented in this work. The matrix formulation included the use of fly ash belite cement (FABC-2-W) and gismondine-type Na-P1 zeolite, both of which are synthesized from fly ash of coal combustion. The efficiency for immobilizing cesium is evaluated from the leaching test ANSI/ANS 16.1-1986 at the temperature of 40ºC, from which the apparent diffusion coefficient of cesium is obtained. Matrices with 100% of FABC-2-W are used as a reference. The integrity of matrices is evaluated by porosity and pore-size distribution from mercury intrusion porosimetry, X-ray diffraction and nitrogen adsorption analyses. Both matrices can be classified as good solidify systems for cesium, specially the FABC-2-W/zeolite matrix in which the replacement of 50% of belite cement by the gismondinetype Na-P1 zeolite caused a decrease of two orders of magnitude of cesium mean Effective Diffusion Coefficient (De) (2.8e−09 cm2/s versus 2.2e−07 cm2/s, for FABC-2-W/zeolite and FABC-2-W matrices, respectively).Funding for the present research was provided by the CAM under Project GR/AMB/0452/2004.Peer reviewed